rying the strata from very high altitudes to very low ones. But no structural features of the plateau country are more truly characteristic than the monoclinal folds and faults. The marginal portions of the plateau country abound in volcanic rocks and extinct volcanoes, while they are almost wholly wanting in the great central areas. The eruptions vary in age from the middle eocene almost to the present, the latest being probably less than three centuries old. This volcanic border is so nearly complete, that, if a geologist were making the circuit of the plateau province, he could so shape his route that for three-fourths of the way he would be treading upon eruptive materials, and pitch his camp upon them every night. The classic group of laccolites known as the Henry Mountains is situated in the northern plateaus, and it is now known that this highly interesting type of eruption has been repeated at various points in the province. But perhaps no geological feature of the plateaus is of greater interest in connection with this monograph than the swells,' of which the San Rafael Swell' is the type; for the Zuni Plateau is a noble example of this structure. There is a considerable number of swells in the plateaus, and they are of great importance by reason of their association with the most impressive features of the region. They are the localities of maximum erosion, - the centres from which the dissolution of the strata, through the wasting of their edges, has proceeded outwards, in ever-expanding circles, one bed or formation following another, until thousands of feet in thickness, and thousands of square miles in area, have been swept away. Along with this denudation has occurred a doming-up of the strata into a broad, gently swelling boss. Captain Dutton's beautiful colored map, and the accompanying sections, bring out the topographic and geologic features of the region to which this monograph especially relates with wonderful distinctness. They show that the Zuñi Plateau is simply a great swell in a vast regional expanse of mesozoic rocks, breaking for a brief space the continuity of that system of strata, and presenting a well-marked monocline on either flank, a long, gentle slope of the strata on the north-east, and a short, abrupt slope on the south-west. From its broad surface the mesozoic has been denuded, leaving the edges of the strata, more or less upturned, to face it round about on all sides in rainbow cliffs. Away from the plateau the strata resume their normal horizontality, and the cretaceous becomes again everywhere the surface of the land. Vast and imposing is the expanse of this mighty cretaceous system. If we could rise in a captive balloon two thousand feet above the Zuñi Plateau, the radius of vision would embrace more than twenty thousand square miles covered with it. Yet this is but a trifle in comparison with its whole extent, which embraces half of the North American continent. Its thickness is equally matter of wonder. Whence came this stupendous mass of material? This is undoubtedly one of the most important and difficult questions in American geology. North-east of the Zuñi Plateau, beyond the noble valley of the San José, rises Mount Taylor. It is a large volcanic cone planted upon a lofty and very extensive mesa of cretaceous strata heavily sheeted over with lava, the lava-cap being seldom less than three hundred feet thick. The cone occupies but a small part of the high platform on which it stands. It is merely the focus and culminating point of a rather large field of volcanic action. It is also clear that the immense cap of lava did not all come from this main orifice, but that the greater part was disgorged from numberless vents scattered over its entire surface, both the concentrated and the diffuse types of volcanic action being well exhibited in the same tract. The great mesa on which Mount Taylor stands is only one of a series, and it forms only a small part of a great volcanic field. From its southern and eastern margins other mesas of similar composition are plainly visible; and it is certain that the sheet of lava once extended, perhaps without a break, across the broad intervening valleys of erosion, for they are now thickly studded with volcanic necks. These necks are ancient vents which have been exposed and left in striking relief by the wearing-away of the softer cretaceous strata over which their flows once spread. They form one of the most interesting and instructive features of the region, and Captain Dutton has described and illustrated them in considerable detail. It is impossible to follow him further in these interest ing descriptive chapters; but we must pass on to the general conclusions. In the stratigraphy of the plateau country there is no fact of greater importance than the general if not complete absence of Devonian and Silurian strata below the carboniferous. In the Grand Cañon of the Colorado the carboniferous beds rest directly and conformably upon the Cambrian; and in the Zuñi Plateau the Cambrian is also wanting, and they repose upon the Archæan. But, although we thus have evidence of considerable areas of dry land in the Far West before carboniferous time, the strata of the latter age present, except where interrupted by subsequent erosion, one almost universal sheet of marine sediments over the whole western country. The whole tenor of the evidence accords well with the inference that the surface of the plateau country during the Jura-Trias coincided very nearly with sea-level, but was continually oscillating from a little above to a little below that level, and vice versa. This is proved by the character of the sediments and the numerous unconformities by erosion, only without any discordance of dip. At one time it was a land area, sustaining a great forest vegetation, through which many species of dinosaurs wandered; at another it was overflowed by the ocean, and received deposits of fine sand, clay, and gypsum. Whatever may be the true explanation, it is a most extraordinary fact that three thousand to four thousand feet of strata were accumulated upon an area of over ninety thousand square miles, and yet the surface of deposition was maintained throughout at approximately the same level. Very similar considerations are presented by the cretaceous system. As in the Jura-Trias, there were alternations of land and sea; and whenever the sea withdrew, the land thus laid bare bloomed with forests and swarmed with dinosaurs. Here we find, for the first time in the West, conditions favorable for the formation of coal. From top to bottom the shaly beds of the cretaceous include coal-seams and carbonaceous layers, while the intervening beds abound in fossil leaves. The carboniferous age of the Appalachians repeated itself here in the closing stages of the mesozoic, and upon a scale of equal if not greater grandeur. An interesting question arises here. How does it happen that coal did not form in the Western Trias also? That vegetation was exuberant in that age is fully attested by the enormous abundance of fossil plants, which are usually silicified. The problem still awaits solution, but certain it is that the Jura-Trias has never yielded in the West a trace of carbonaceous matter. Its trees and shrubs have turned into stone instead of coal. The source of the detritus forming the mesozoic strata of the West is found chiefly in the Great Basin of Utah, Nevada, etc. The fact is general that these strata grow thinner from west to east, indicating a western origin for its sediments. But the stupendous volume of the sediments in the United States and British America also indicates that they came from a source which was much more extensive than any island; in short, from some continental area, including the Great Basin, and having a shore-line many hundreds of miles long, with numerous large rivers discharging sand and silt. The movements which ultimately isolated the plateau province, and gave it its distinctive history and development, began in the Laramie period; and during eocene time its area was a vast inland lake with an outlet. In this lake eocene sediments were deposited to a maximum thickness of five thousand feet toward the north, and thinning southward, indicating that they were derived mainly from the Rocky, Uinta, and Wasatch ranges, which were then in existThe plateau lake finally disappeared in the miocene period, and thus closed the long period of almost continuous deposition which began in early carboniferous time, and during which from ten thousand to fifteen thousand feet of sediments were accumulated. ence. All this region proclaims an ancient erosion far more vigorous than the present. This is seen in the wide, eroded valleys, fit for the passage of great rivers, but vacant now of flowing waters, their troughs half filled with alluvium, and the grass growing over their flood-plains. We are obliged to refer this erosion to the miocene, and the great elevation of the country which followed it probably occurred during the pliocene. |