Laacher See
Fig. 23.—Plan and Section of the Laacher See, a lake on the borders of the Eifel, occupying the crater of an old volcano.—G. Gravel and volcanic sand forming banks of the lake and rim of old crater; L. Sheet of trachytic lava with columnar structure; B. Basaltic dyke; S. Devonian slate, etc.

The Laacher See is a lake of an oval form, over an English mile in the shorter diameter, and surrounded by high banks of volcanic sand, gravel, and scoriæ, except on the east side, where cliffs of clay-slate, in a nearly vertical position, and striking nearly E.W., may be observed. Its depth from the surface of the water is 214 feet.[10] The ashes of the encircling banks contain blocks of slate and lava which have been torn from the sides of the orifice or neck of the volcano and blown into the air; and there can be no doubt that the ashes and volcanic gravel is the result of very recent eruptions.

At the east side of the lake we find a stream of scoriaceous lava of a purple or reddish colour, highly vesicular, and containing crystals of mica; but the most important lava-stream is that which has taken a southerly direction from the crater of the Laacher See towards Nieder Mendig and Mayen, for a distance of about six miles. This great stream is covered throughout half its distance by beds of volcanic ash and lapilli, but emerges into the air at a distance of about two miles from the edge of the crater (see Fig. 23), and was formerly extensively quarried in underground caverns for millstones. Here the rock is a vesicular trachyte, of a greyish colour, solidified in vertical columns of hexagonal form, about four feet in diameter, and traversed by transverse joint planes. These quarries have been worked from the time of the Roman occupation of the country; and, before the introduction of iron or steel rollers for grinding corn, millstones were exported to all parts of Europe and the British Isles from this quarry.[11]

The district around the Laacher See is covered by laminated ejecta of the old volcano, probably of subaërial origin, through which bosses of the fundamental slate peer up at intervals, while the surface is diversified by several truncated cones.

(g.) Trass of the Brühl Valley.—The Brühl Valley, which unites with that of the Rhine at the town of that name, and drains the northern side of the volcanic region, has always been regarded with much interest by travellers for the presence of a deposit of "trass" with which it is partially filled. The origin of this valley was pre-volcanic, as it is hewn out of the slaty rocks of the district. But at a later period it became filled with volcanic mud (tuffstein), out of which the stream has made for itself a fresh channel. The source of this mud is considered by Hibbert[12] to have been the old volcano of the Lummerfeld, which, after becoming dormant, was filled with water, and thus became a lake. At a subsequent period, however, a fresh eruption took place near the edge of the lake, resulting in the remarkable ruptured crater known as the Kunksköpfe, which rises about four miles to the north of the Laacher See. The eruptions of this volcano appear to have displaced the mud of the Lummerfeld, causing it to flow down into the deep gorge of the Brühl, which it completely filled, as stated above.

On walking down the valley one may sometimes see the junction of the tuff with the slate-rock which enfolds it. The tuff consists of white felspathic mud, with fragments of slate and lava, reaching a depth in some places of 150 feet. After it has been quarried it is ground in mills, and used for cement stone under the name of trass. It is said to resemble the volcanic mud by which Herculaneum was overwhelmed during the first eruption of Vesuvius, and which was produced by the torrents of rain mixing with the ashes as they were blown out of the volcano.

Sufficient has probably now been written regarding the dormant, or recently extinct, volcanic districts of Europe to give the reader a clear idea regarding their nature and physical structure. Other districts might be added, such as those of Central Germany, Hungary, Transylvania, and Styria; but to do so would be to exceed the proposed limits of this work; and we may therefore pass on to the consideration of the volcanic region of Syria and Palestine, which adjoins the Mediterranean district we have considered in a former page.

[1] Daubeny, loc. cit., p. 71. The geology of this region has had many investigators, of whom the chief are Steininger, Erloschenen Vulkane in der Eifel (1820); Hibbert, Extinct Volcanoes of the Basin of Neuwied, 1832; Nöggerath, Das Gebirge im Rheinland, etc., 4 vols.; Horner, "On the Geology of Bonn," Transactions of the Geological Society, London, vol. iv.

[2] The views of Dr. Hibbert are not inconsistent with those of the late Sir A. Ramsay, on "The Physical History of the Valley of the Rhine," Quart. Jour. Geol. Soc., vol. xxx. (1874).

[3] Von Dechen, Geog. Beschreib. des Siebengebirges am Rhein (Bonn, 1852).

[4] Hibbert, loc. cit., p. 18.

[5] Horner, "Geology of Environs of Bonn," Transactions of the Geological Society, vol. iv., new series.

[6] H. von Dechen, Geog. Führer in das Siebengebirge am Rhein (Bonn, 1861).

[7] Ibid., p. 191.

[8] Dr. Hibbert's work is illustrated by very carefully drawn and accurate views of some of the old cones and craters of this district, accompanied by detailed descriptions.

[9] The lava of Schorenberg, near Rieden, is interesting from the fact, stated by Zirkel, that it contains leucite, nosean, and nephelin.—Die Mikros. Beschaf. d. Miner. u. Gesteine, p. 154 (1873).

[10] Hibbert, loc. cit., p. 23.

[11] At the time of the author's visit the underground caverns, which are deliciously cool in summer, were used for the storage of the celebrated beer brewed by the Moravians of Neuwied.

[12] Hibbert, loc. cit., p. 129.


PART III.

DORMANT OR MORIBUND VOLCANOES OF OTHER PARTS OF THE WORLD.


CHAPTER I.

DORMANT VOLCANOES OF PALESTINE AND ARABIA.

(a.) Region east of the Jordan and Dead Sea.—The remarkable line of country lying along the valley of the Jordan, and extending into the great Arabian Desert, has been the seat of extensive volcanic action in prehistoric times. The specially volcanic region seems to be bounded by the depression of the Jordan, the Dead Sea, and the Arabah as far south as the Gulf of Akabah; for, although Safed, lying at the head of the Sea of Galilee on the west of the Jordan valley, is built on a basaltic sheet, and is in proximity to an extinct crater, its position is exceptional to the general arrangement of the volcanic products which may be traced at intervals from the base of Hermon into Central Arabia, a distance of about 1000 miles.[1]

The tract referred to has been described at intervals by several authors, of whom G. Schumacher,[2] L. Lartet,[3] Canon Tristram,[4] M. Niebuhr,[5] and C. M. Doughty[6] may be specially mentioned in this connection.

The most extensive manifestations of volcanic energy throughout this long tract of country appear to be concentrated at its extreme limits. At the northern extremity the generally wild and rugged tract of the Jaulân and Haurân, called in the Bible Trachonitis, and still farther to the eastward the plateau of the Lejah, with its row of volcanic peaks sloping down to the vast level of Bashan, is covered throughout nearly its whole extent by great sheets of basaltic lava, above which rise at intervals, and in very perfect form, the old crater-cones of eruption. A similar group of extinct craters with lava-flows has been described and figured by a recent traveller, Mr. C. M. Doughty, in parts of Central Arabia. The general resemblance of these Arabian volcanoes to those of the Jaulân is unquestionable; and as they are connected with each other by sheets of basaltic lava at intervals throughout the land of Moab, it is tolerably certain that the volcanoes lying at either end of the chain belong to one system, and were contemporaneously in a state of activity.

(b.) Geological Conditions.—Before entering any further into particulars regarding the volcanic phenomena of this region, it may be desirable to give a short account of its geological structure, and the physical conditions amongst which the igneous eruptions were developed.

Down to the close of the Eocene period the whole region now under consideration was occupied by the waters of the ocean. The mountains of Sinai were islands in this ocean, which had a very wide range over parts of Asia, Africa, and Europe. But at the commencement of the succeeding Miocene stage the crust was subjected to lateral contraction, owing to which the ocean bed was upraised. The strata were flexured, folded, and often faulted and fissured along lines ranging north and south, the great fault of the Jordan-Arabah valley being the most important. At this period the mountains of the Lebanon, the table-lands of Judæa and of Arabia, formed of limestone, previously constituting the bed of the ocean during the Eocene and Cretaceous periods, were converted into land surfaces. Along with this upheaval of the sea-bed there was extensive denudation and erosion of the strata, so that valleys were eroded over the subaërial tracts, and the Jordan-Arabah valley received its primary form and outline.

Up to this time there does not appear to have been any outbreak of volcanic forces; but with the succeeding Pliocene period these came into play, and eruptions of basaltic lava took place along rents and fissures in the strata, while craters and cones of slag, scoriæ, and ashes were thrown up over the region lying to the east of the Sea of Galilee and the sources of the Jordan on the one hand, and the central parts of the great Arabian Desert on the other. These eruptions, probably intermittent, continued into the succeeding Glacial or Pluvial period, and only died out about the time that the earliest inhabitants appeared on the scene.

(c.) The Jaulân and Haurân.—This tract is bounded by the valley of the Jordan and the Sea of Galilee on the west, from which it rises by steep and rocky declivities into an elevated table-land, drained by the Yarmûk (Hieromax), the Nahr er Rukkâd, and other streams, which flow westwards into the Jordan along deep channels in which the basaltic sheets and underlying limestone strata are well laid open to view.

On consideration it seems improbable that the great sheets of augitic lava, such as cover the surface of the land of Bashan, are altogether the product of the volcanic mountains which appear to be confined to special districts in this wide area. Some of the craters do indeed send forth visible lava-streams, but they are insignificant as compared with the general mass of the plateau-basalts; and the crater-cones themselves appear in some cases to be posterior to the platforms of basalt from which they rise. It is very probable, therefore, that the lavas of this region have, in the main, been extruded from fissures of eruption at an early period, and spread over the surface of the country in the same manner as those of the Snake River region, and the borders of the Pacific Ocean of North America, and possibly of the Antrim Plateau in Ireland, afterwards to be described.

The volcanic hills which rise above the plateau are described in detail by Schumacher. Of these, Tell Abû Nedîr is the largest in the Jaulân. It reaches an elevation of 4132 feet above the Mediterranean Sea, and 1710 feet above the plain from which it rises; the circumference of its base is three miles, and the rim of the crater itself, which is oval in form, is 1331 yards in its larger diameter. The interior is cultivated by Circassians, and is very fruitful; the walls descend at an angle of about 30° on the inside, the exterior slope of the mountain being about 22°. The cone seems to be formed chiefly of scoriæ, and the lava-stream, which issues forth from the interior, forms a frightfully stony and lacerated district.[7]

Craters in the Jaulân
Fig. 24.—Extinct Craters in the Jaulân, north-east from the Sea of Galilee, called Tell Abû en Nedâ and Tell el Urâm, with a central cone.—(After Schumacher.)

Another remarkable volcano is the Tell Abû en Nedâ (Fig. 24). This is a double crater, with a cone (probably of cinders) rising from the interior of one of them. The highest point of the rim of one of the craters reaches a level of 4042 feet above the sea. A lava-stream issues forth from Abû en Nedâ, and unites with another from a neighbouring volcano.

Tell el Ahmâr is a ruptured crater of imposing aspect, reaching an elevation of 4060 feet, and sending forth a lava-current, which falls in regular terraces from the outlet towards the west and north.

The ruptured crater of Tell el Akkasheh, which reaches a height of 3400 feet, has a less forbidding aspect than the greater number of the extinct volcanoes of this region, owing to the fact that its sides are covered by oaks, which attain to magnificent proportions along the summit. Numerous other volcanic hills occur in this district, but the most remarkable is that called Tell el Farras (the Hill of the Horse). It is an isolated mountain, visible from afar, and reaches an elevation of 3110 feet, or nearly 800 feet above the surrounding plain. The oval crater of this volcano opens towards the north, and has a depth of 108 feet below the edge, with moderately steep sloping sides (17°-32°), while the slope of the exterior, at first steep, gradually lessens to 20°-21°. These slopes are covered with reddish or yellowish slag. The above examples will probably suffice to afford the reader a general idea of the size and form of the volcanoes in this little known region.

It has been stated above that the great lava-floods have probably been poured forth intermittently. The statement receives confirmation from the observations of Canon Tristram, made in the valley of the Yarmûk.[8] This impetuous torrent rushes down a gorge, sometimes having limestone on one side and a wall of basalt on the other. This is due to the fact that the river channel had been eroded before the volcanic eruptions had commenced; but on the lava-stream reaching the channel, it naturally descended towards the valley of the Jordan along its bed, displacing the river, or converting it into clouds of steam. Subsequently the river again hewed out its channel, sometimes in the lava, sometimes between this rock and the chalky limestone. But, in addition to this, it has been observed that there is a bed of river gravel interposed between two sheets of basalt in the Yarmûk ravine; showing that after the first flow of that molten rock the river reoccupied its channel, which was afterwards invaded by another molten lava-stream, into which the waters have again furrowed the channel which they now occupy. The basaltic sheets descend under the waters of the Sea of Galilee on the east side, and were probably connected with those of Safed, crossing the Jordan valley north of that lake; owing to this the waters of the Lake of Merom (Huleh) were pent up, and formerly covered an extensive tract, now formed of alluvial deposits.

(d.) Land of Moab.—Proceeding southwards into the Land of Moab, the volcanic phenomena are here of great interest. Extensive sheets of basaltic lava, described as far back as 1807 by Seetzen, and more recently by Lartet and Tristram, are found at intervals between the Wâdies Mojib (Arnon) and Haidan. On either side of the Mojib, cliffs of columnar basalt are seen capping the beds of white Cretaceous limestone, while a large mass has descended into the W. Haidan between cliffs of limestone and marl on either hand.

Around Jebel Attarus—a dome-shaped hill of limestone—a sheet of basaltic lava has been poured, and has descended the deep gorge of the Zerka Maïn, which enters the Dead Sea some 2000 feet below. This gorge had been eroded before the basaltic eruption, so that the stream of molten lava took its course down the bed of this stream to the water's edge, and grand sections have been laid bare by subsequent erosion along the banks. Pentagonal columns of black basalt form perpendicular walls, first on one side, then on the other; while considerable masses of scoriæ, peperino, and breccia appear at the head of the glen, probably marking the orifice of eruption. Other eruptions of basalt occur, one at Mountar ez Zara, to the south of Zerka Maïn, and another at Wady Ghuweir, near the north-eastern end of the Dead Sea. There are no lava-streams on the western side of the Ghor, or of the Dead Sea.[9]

The outburst of the celebrated thermal springs of Callirrhoë, together with nine or ten others, along the channel of the Zerka Maïn, is a circumstance which cannot be dissociated from the occurrence of basaltic lava at this spot. In a reach of three miles, according to Tristram, there are ten principal springs, of which the fifth in descent is the largest; but the seventh and eighth, about half a mile lower down, are the most remarkable, giving forth large supplies of sulphurous water. The tenth and last is the hottest of all, indicating a temperature of 143° Fahr. Thus it would appear that the heat increases with the depth from the upper surface of the table-land; a result which might be expected, supposing the heated volcanic rocks to be themselves the source of the high temperature. To a similar cause may be attributed the hot-springs of Hammath, near Tiberias, and those of the Yarmûk near its confluence with the Jordan. Some of these and other springs break out along, or near, the line of the great Jordan-Arabah fault which ranges throughout the whole extent of this depression, from the base of Hermon to the Gulf of Akabah, generally keeping close to the eastern margin of the valley.

(e.) The Arabian Desert.—The basaltic lava-floods occupy a very large extent of the Arabian Desert, from El Hisma (lat. 27° 35' N.) to the neighbourhood of Mecca on the south, a distance of about 440 miles, with occasional intervals. The lava-sheets are called "Harras" (or "Harrat"), one of which, Harrat Sfeina, terminates about ten miles north of Mecca. The lava-sheets rest sometimes on the red sandstone, at other times, on the granite and other crystalline rocks of great geological antiquity. In addition to the sheets of basalt, numerous crater-cones rise from the basaltic platform at a level of 5000 feet above the sea, and two volcanic mountains, rising far to the west of the principal range, called respectively Harrât Jeheyma and H. Rodwa, almost overlook the coast of the Red Sea.[10]

(f.) Age of the Volcanic Eruptions.—It is very clear that the first eruptions, producing the great basaltic sheets of Moab and Arabia, occurred after the principal features of the country had been developed. The depression of the Jordan-Arabah valley, the elevation of the eastern side of this valley along the great fault line, and the channels of the principal tributary streams, such as those of the Yarmûk and Zerka Maïn, all these had been eroded out before they were invaded by the molten streams of lava. Now, as these physical features were developed and sculptured out during the Miocene period, as I have elsewhere shown to be the case,[11] we may with great probability refer the volcanic eruptions to the geological epoch following—namely, the Pliocene. How far downwards towards the historic period the eruptions continued is not so certain. Dr. Daubeny, quoting several passages from the Old Testament prophets,[12] says it might be inferred that volcanoes were in activity even so late as to admit of their being included within the limits of authentic history. The poetic language and imagery used in these passages by the prophets certainly lends a probability to this view, but nothing more. On the other hand, these regions have suffered through many centuries from the secondary effects of seismic action and subterranean forces, and earthquake shocks have laid in ruins the great temples and palaces of Palmyra, Baalbec, and other cities of antiquity. The same uncertainty regarding the time at which volcanic action died out, with reference to the appearance of man on the scene, hangs over the region of Arabia and Syria, as we have seen to be the case in reference to the extinct volcanoes of Auvergne, the Eifel, and the Lower Rhine. In all these cases the commencement and close of eruptive action appear to have been very much about the same period—namely, the Miocene period on the one hand, and that at which man entered upon the scene on the other; but in the case of Syria and Western Palestine, the close of the volcanic period may have been somewhat more than 2000 B.C.

[1] Lake Phiala, near the Lake of Huleh, is also situated to the west of the Jordan valley. Its origin, according to Tristram, is volcanic.

[2] Schumacher, "The Jaulân," Quarterly Statement of the Palestine Exploration Fund, 1886 and 1888; and Across the Jordan, London, 1886.

[3] Lartet, Voyage d'Exploration de la mer Morte (Géologie), Paris, 1880.

[4] Tristram, Land of Moab, London, 1873; and Land of Israel, 1866.

[5] Niebuhr, Beschreibung von Arabien, 1773.

[6] C. M. Doughty, Arabia Deserta, 2 vols., 1888. A generalised account of this volcanic region by the author will be found in the "Memoir on the Physical Geology of Arabia Petræa, and Palestine," Palestine Exploration Fund, 1887.

[7] Schumacher, loc. cit., p. 248.

[8] Land of Israel, p. 461.

[9] "Geology of Arabia Petræa, and Palestine," Memoirs of the Palestine Exploration Fund, p. 95.

[10] Doughty, loc. cit., vol. i., plate vi., p. 416. An excellent geological sketch map accompanies this work.

[11] "Memoir of the Geology of Arabia Petræa, and Palestine," chap. vi. p. 67.

[12] Nahum, i. 5, 6; Micah, i. 3, 4; Isaiah, lxiv. 1-3; Jeremiah, l. 25.


CHAPTER II.

THE VOLCANIC REGIONS OF NORTH AMERICA.

(a.) Contrast between the Eastern and Western Regions.—In no point is there a more remarkable contrast between the physical structure of Eastern and Western America than in the absence of volcanic phenomena in the former and their prodigious development in the latter. The great valley of the Mississippi and its tributaries forms the dividing territory between the volcanic and non-volcanic areas; so that on crossing the high ridges in which the western tributaries of America's greatest river have their sources, and to which the name of the "Rocky Mountains" more properly belongs, we find ourselves in a region which, throughout the later Tertiary times down almost to the present day, has been the scene of volcanic operations on the grandest scale; where lava-floods have been poured over the country through thousands of square miles, and where volcanic cones, vying in magnitude with those of Etna, Vesuvius, or Hecla, have established themselves. This region, generally known as "The Great Basin," is bounded on the west by the "Pacific Range" of mountains, and includes portions of New Mexico, Arizona, California, Nevada, Utah, Colorado, Idaho, Oregon, Wyoming, Montana, and Washington. To the south it passes into the mountainous region of Mexico, also highly volcanic; and thence into the ridge of Panama and the Andes. It cannot be questioned but that the volcanic nature of the Great Basin is due to the same causes which have originated the volcanic outbursts of the Andes; but, from whatever cause, the volcanic forces have here entered upon their secondary or moribund stage. In the Yellowstone Valley, geysers, hot springs, and fumaroles give evidence of this condition. In other districts the lava-streams are so fresh and unweathered as to suggest that they had been erupted only a few hundred years ago; but no active vent or crater is to be found over the whole of this wide region. A few special districts only can here be selected by way of illustration of its special features in connection with its volcanic history.

(b.) The Plateau Country of Utah and Arizona.—This tract, which is drained by the Colorado River and its tributaries, is bounded on the north by the Wahsatch range, and extends eastwards to the base of the Sierra Nevada. Round its margin extensive volcanic tracts are to be found, with numerous peaks and truncated cones—the ancient craters of eruption—of which Mount San Francisco is the culminating eminence. South of the Wahsatch, and occupying the high plateaux of Utah, enormous masses of volcanic products have been spread over an area of 9000 square miles, attaining a thickness of between 3000 and 4000 feet. The earlier of these great lava-floods appear to have been trachytic, but the later basaltic; and in the opinion of Captain Dutton, who has described them, they range in point of time from the Middle Tertiary (Miocene) down to comparatively recent times.

(c.) The Grand Cañon.—To the south of the high plateaux of Utah are many minor volcanic mountains, now extinct; and as we descend towards the Grand Cañon of Colorado we find numerous cinder-cones scattered about at intervals near the cliffs.[1] Extensive lava-fields, surmounted by cinder-cones, occupy the plateau on the western side of the Grand Cañon; and, according to Dutton, the great sheets of basaltic lava, of very recent age, which occupy many hundred square miles of desert, have had their sources in these cones of eruption.[2] Crossing to the east of the Grand Cañon, we find other lava-floods poured over the country at intervals, surmounted by San Francisco—a volcanic mountain of the first magnitude—which reaches an elevation, according to Wheeler, of 12,562 feet above the ocean. It has long been extinct, and its summit and flanks are covered with snow-fields and glaciers. Other parts of Arizona are overspread by sheets of basaltic lava, through which old "necks" of eruption, formed of more solid lava than the sheets, rise occasionally above the surface, and are prominent features in the landscape.

Further to the eastward in New Mexico, and near the margin of the volcanic region, is another volcanic mountain little less lofty than San Francisco, called Mount Taylor, which, according to Dutton, rises to an elevation of 11,390 feet above the ocean, and 8200 feet above the general level of the surrounding plateau of lava. This mountain forms the culminating point of a wide volcanic tract, over which are distributed numberless vents of eruption. Scores of such vents—generally cinder-cones—are visible in every part of the plateau, and always in a more or less dilapidated condition.[3] Mount Taylor is a volcano, with a central pipe terminating in a large crater, the wall of which was broken down on the east side in the later stage of its history.

Mount Shasta
Fig. 25.—Mount Shasta (14,511 feet), a snow-clad volcanic cone in California, with Mount Shastina, a secondary crater, on the right; the valley between is filled with glacier-ice.—(After Dutton).

(d.) California.—Proceeding westwards into California, we are again confronted with volcanic phenomena on a stupendous scale. The coast range of mountains, which branches off from the Sierra Nevada at Mount Pinos, on the south, is terminated near the northern extremity of the State by a very lofty mountain of volcanic origin, called Mount Shasta, which attains an elevation of 14,511 feet (see Fig. 25). This mountain was first ascended by Clarence King in 1870,[4] and although forming, as it were, a portion of the Pacific Coast Range, it really rises from the plain in solitary grandeur, its summit covered by snow, and originating several fine glaciers.

The summit of Mount Shasta is a nearly perfect cone, but from its north-west side there juts out a large crater-cone just below the snow-line, between which and the main mass of the mountain there exists a deep depression filled with glacier ice. This secondary crater-cone has been named Mount Shastina, and round its inner side the stream of glacier ice winds itself, sometimes surmounting the rim of the crater, and shooting down masses of ice into the great caldron. The length of this glacier is about three miles, and its breadth about 4000 feet. Another very lofty volcanic mountain is Mount Rainier, in the Washington territory, consisting of three peaks of which the eastern possesses a crater very perfect throughout its entire circumference. This mountain appears to be formed mainly of trachytic matter. Proceeding further north into British territory, several volcanic mountains near the Pacific Coast are said to exhibit evidence of activity. Of these may be mentioned Mount Edgecombe, in lat. 57°.3; Mount Fairweather, lat. 57°.20 which rises to a height of 14,932 feet; and Mount St. Elias, lat. 60°.5, just within the divisional line between British and Russian territory, and reaching an altitude of 16,860 feet. This, the loftiest of all the volcanoes of the North American continent, except those of Mexico, may be considered as the connecting link in the volcanic chain between the continent and the Aleutian Islands.[5]

(e.) Lake Bonneville.—Returning to Utah we are brought into contact with phenomena of special interest, owing to the inter-relations of volcanic and lacustrine conditions which once prevailed over large tracts of that territory. The present Great Salt Lake, and the smaller neighbouring lakes, those called Utah and Sevier, are but remnants of an originally far greater expanse of inland water, the boundaries of which have been traced out by Mr. C. K. Gilbert, and described under the name of Lake Bonneville.[6] The waters of this lake appear to have reached their highest level at the period of maximum cold of the Post-Pliocene period, when the glaciers descended to its margin, and large streams of glacier water were poured into it. Eruptions of basaltic lava from successive craters appear to have gone on before, during, and after the lacustrine epochs; and the drying up of the waters over the greater extent of their original area, now converted into the Sevier Desert, and their concentration into their present comparatively narrow basins, appears to have proceeded pari passu with the gradual extinction of the volcanic outbursts. Two successive epochs of eruption of basalt appear to have been clearly established—an earlier one of the "Provo Age," when the lava was extruded from the Tabernacle craters, and a later epoch, when the eruptions took place from the Ice Spring craters. The oldest volcanic rock appears to be rhyolite, which peers up in two small hills almost smothered beneath the lake deposits. Its eruption was long anterior to the lake period. On the other hand, the cessation of the eruptions of the later basaltic sheets is evidently an event of such recent date that Mr. Gilbert is led to look forward to their resumption at some future, but not distant, epoch. As he truly observes, we are not to infer that, because the outward manifestations of volcanic action have ceased, the internal causes of those manifestations have passed away. These are still in operation, and must make themselves felt when the internal forces have recovered their exhausted energies; but perhaps not to the same extent as before.

(f.) Region of the Snake River.—The tract of country bordering the Snake River in Idaho and Washington is remarkable for the vast sheets of plateau-basalt with which it is overspread, extending sometimes in one great flood farther than the eye can reach, and what is still more remarkable, they are often unaccompanied by any visible craters or vents of eruption. In Oregon the plateau-basalt is at least 2,000 feet in thickness, and where traversed by the Columbia River it reaches a thickness of about 3,000 feet. The Snake and Columbia rivers are lined by walls of volcanic rock, basaltic above, trachytic below, for a distance of, in the former, one hundred, in the latter, two hundred, miles. Captain Dutton, in describing the High Plateau of Utah, observes that the lavas appear to have welled up in mighty floods without any of that explosive violence generally characteristic of volcanic action. This extravasated matter has spread over wide fields, deluging the surrounding country like a tide in a bay, and overflowing all inequalities. Here also we have evidence of older volcanic cones buried beneath seas of lava subsequently extruded.

(g.) Fissures of Eruption.—The absence, or rarity, of volcanic craters or cones of eruption in the neighbourhood of these great sheets has led American geologists to the conclusion that the lavas were in many cases extruded from fissures in the earth's crust rather than from ordinary craters.[7] This view is also urged by Sir A. Geikie, who visited the Utah region of the Snake River in 1880, and has vividly described the impression produced by the sight of these vast fields of basaltic lava. He says, "We found that the older trachytic lavas of the hills had been deeply trenched by the lateral valleys, and that all these valleys had a floor of black basalt that had been poured out as the last of the molten materials from the now extinct volcanoes. There were no visible cones or vents from which these floods of basalt could have proceeded. We rode for hours by the margin of a vast plain of basalt stretching southward and westward as far as the eye could reach.... I realised the truth of an assertion made first by Richthofen,[8] that our modern volcanoes, such as Vesuvius and Etna, present us with by no means the grandest type of volcanic action, but rather belong to a time of failing activity. There have been periods of tremendous volcanic energy, when instead of escaping from a local vent, like a Vesuvian cone, the lava has found its way to the surface by innumerable fissures opened for it in the solid crust of the globe over thousands of square miles."[9]

(h.) Volcanic History of Western America.—The general succession of volcanic events throughout the region of Western America appears to have been somewhat as follows:—[10]

The earliest volcanic eruptions occurred in the later Eocene epoch and were continued into the succeeding Miocene stage. These consisted of rocks moderately rich in silica, and are grouped under the heads of propylite and andesite. To these succeeded during the Pliocene epoch still more highly silicated rocks of trachytic type, consisting of sanidine and oligoclase trachytes. Then came eruptions of rhyolite during the later Pliocene and Pleistocene epochs; and lastly, after a period of cessation, during which the rocks just described were greatly eroded, came the great eruptions of basaltic lava, deluging the plains, winding round the cones or plateaux of the older lavas, descending into the river valleys and flooding the lake beds, issuing forth from both vents and fissures, and continuing intermittently down almost into the present day—certainly into the period of man's appearance on the scene. Thus the volcanic history of Western America corresponds remarkably to that of the European regions with which we have previously dealt, both as regards the succession of the various lavas and the epochs of their eruption.

(i.) The Yellowstone Park.—The geysers and hot springs of the Yellowstone Park, like those in Iceland and New Zealand, are special manifestations of volcanic action, generally in its secondary or moribund stage. The geysers of the Yellowstone occur on a grand scale; the eruptions are frequent, and the water is projected into the air to a height of over 200 feet. Most of these are intermittent, like the remarkable one known as Old Faithful, the Castle Geyser, and the Giantess Geyser described by Dr. Hayden, which ejects the water to a height of 250 feet. The geyser-waters hold large quantities of silica and sulphur in solution, owing to their high temperature under great pressure, and these minerals are precipitated upon the cooling of the waters in the air, and form circular basins, often gorgeously tinted with red and yellow colours.[11]