3i8 



SCIENCE. 



[Vol. X. No. 256 



tying the strata from very high altitudes to very low ones. But no 

 structural features of the plateau country are more truly character- 

 istic 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 com- 

 plete, 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 Zuiii 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 re- 

 gion to which this monograph especially relates with wonderful 

 distinctness. They show that the Zuni 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 re- 

 sume their normal horizontality, and the cretaceous becomes again 

 everywhere the surface of the land. Vast and imposing is the ex- 

 panse of this mighty cretaceous system. If we could rise in a 

 captive balloon two thousand feet above the Zuiii Plateau, the radi- 

 us 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 conti- 

 nent. 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 Zuiii Plateau, beyond the noble valley of the 

 San Jose, 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 cul- 

 minating 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 com- 

 position are plainly visible ; and it is certain that the sheet of lava 

 once extended, perhaps without a break, across the broad interven- 

 ing valleys of erosion, for they are now thickly studded with vol- 

 canic necks. These necks are ancient vents which have been ex- 

 posed 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 Button has described and illustrated them in consider- 

 able detail. It is impossible to follow him further in these interest- 



ing descriptive chapters ; but we must pass on to the general con- 

 clusions. 



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 Cafion of the Colorado the carboniferous beds rest directly 

 and conformably upon the Cambrian ; and in the Zufii Plateau the 

 Cambrian is also wanting, and they repose upon the Archaean. 

 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 coin- 

 cided 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 un- 

 conformities 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 sys- 

 tem. 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 forma- 

 tion 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 Appa- 

 lachians 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 stupen- 

 dous 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 conti- 

 nental area, including the Great Basin, and having a shore-line 

 many hundreds of miles long, with numerous large rivers dischar- 

 ging 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 exist- 

 ence. The 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 accumu- 

 lated. 



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. 



