510 INDEX TO THE STEATIGKAPHY OF NOETH AMEEICA. 



Section of coal beds at Farmville, N.,0. 

 Slate roof. 



Coal, good 3 feet to 3 feet 2 inches. 



Black band and elate 1 foot 8 inches to 3 feet. 



Coal, poor, shaly 1 foot to 2 feet. 



Pure clay floor. 



I-K 12-13. TTTAH, COLORADO, NEW MEXICO, AND ARIZONA. 



Along the fortieth parallel so-called Triassic rocks were originally distinguished 

 by King and his associates/"^'^ whose observations are thus summed up: 



In the region of the Eocky Mountains we have seen that the Trias frequently overlaps 

 the older rocks and comes directly into nonconformable contact with the great Archean islands 

 that now form the three ranges of the Eocky Mountain system in our latitude. The Trias is 

 in general a series of sandstones; the upper half is always of lighter colors than the lower half 

 and is always intercalated more or less with beds of dolomitic limestone and gypsum. The 

 series varies from 300 to 1,000 feet in thickness. Wherever it stands at a high dip, it is most 

 compressed in thickness and most compacted in lithological character. Wherever its position 

 approaches horizontality, the texture of the rock is that of a loose, friable sediment. The 

 lower half of the series is usually from brick to vermilion red and buff, with occasional excep- 

 tions of white and brilhant vermilion. The intercalated dolomitic and gypsum beds are never 

 continuous but are shallow deposits of no' great lateral extension. On approaching the Archean 

 rocks the Trias have always more or less local conglomerates, derived directly from the shores 

 against which they abut. There is considerable variabihty in color, in thickness, and in special 

 arrangement and sequence of the sediments. From 1,000 feet maximum in the region of the 

 Eocky Mountains the deposit thickens in passing westward, until, in the neighboihood of the 

 eastern part of the Uinta, it is fully 2,000 to 2,500 feet thick. The division between the lower 

 dark-red member and the upper buff or white member is much more distinct in the Uinta 

 region than to the east. Here, however, are still the intercalated gypsums and dolomites 

 in the upper half of the series, the gypsum sometimes reaching 40 feet of pure white crystalline 

 sulphate. There are also in the Uinta considerable intercalations of clayey matter, which 

 are rare in Colorado. 



Passing still farther westward, against the Wahsatch, there is again a noticeable diminu- 

 tion of thickness and a corresponding increase of stony compactness. Under the microscope 

 no single specimen was observed that had not a considerable amount of carbon and a trace 

 of crystals of carbonate of lime. In approaching the Wahsatch, also, there is a considerable 

 increase of conglomerates. This constitutes another argument indicating the approach of 

 a land mass to the west, whence detritus is derived. But one fossil, a new species, was found 

 in the entire Triassic series of the east, and that was obtained from one of the limestone beds — 

 a greenish-drab lithographic limestone — a little above the middle of the series, on the south 

 flank of the Uinta. That fossil had a distinctly upper Triassic or Jurassic facies. The upper 

 horizons, especially the uppermost member of aU, varying from 200 feet in Colorado to 600 in 

 the Uinta and sometimes more than that upon the flanks of the Wahsatch, is characterized by 

 remarkable cross stratification which is prominent over most of the exposed area east of the 

 Wahsatch. The flow and plunge structure is developed in a perfection rarely seen, the plane 

 of the cross stratification often inclining to the true bedding planes at an angle of 30° to 35°. 



The upper half, bearing irregular sheets of gypsum and of dolomitic limestone, is always 

 directly conformably overlain by the Jurassic beds, which, when first seen on the east flank 

 of Colorado Eange, vary from 250 to 275 feet in thiclaiess, and increase steadily until on the 

 flanks of the Wahsatch they have reached fully 1,800 feet. There is a very great physical 

 contrast between the general character of the materials of the Triassic and the Jurassic series. 

 The former is, on the whole, free from lime, except in the sulphate and dolomitic beds, and 

 is with the exception of certain parts of the Uinta rather free from intercalated clays. On the 

 other hand, the Jurassic, in the Eocky Mountain region, is entirely made up of soft clays; 

 argillaceous and calcareous marls, and thin intercalations of fine lithographic limestone. In 



