GEOLOGICAL SUKVEY OF THE TEKRITOEIES. 133 



of a thick bed of yellowish sac d stone. At another locality a few yards 

 distant a small stream, in cutting its way through this ridge, revealed 

 alternate layers of ash-colored and yellow arenaceous clay, with some 

 hard beds of sandstone, inclining 55°. A portion of these beds are 

 probably Jurassic. We have here an interval in the harder beds between 

 the high sandstone ridge and the sandstones of No. 1, filled up with 

 yielding clays and sands, which I estimated at from six hundred to 

 seven hundred feet in thickness. Then come the sandstones of No. 1, 

 and the gray limestones and shales of No. 2, and the chalky marls of 

 No. 3, which are plainly visible with about the same dip. Although the 

 grass covers the surface to such an extent that the upper cretaceous 

 beds are not exposed, yet it is safe to suppose that the entire series of 

 cretaceous formations, as known along the flanks of the mountains, 

 exist here. 



There is ample room, also, for a great thickness of the tertiary beds, 

 and the evidence is quite clear that a large portion of the sandstones, 

 clays, and doubtless beds of coal, of the tertiary x)eriod exist in the 

 enormous plateau or table-like bench which extends down the Boulder 

 y alley from the foot of the mountains. ^ 



The amount of loose drift material is enormous, scattered not only 

 over the surface, but concealing to a great extent the underlying basis 

 rocks. There is, therefore, some reason to believe that the coal may 

 yet be found in the valley under South Boulder Creek and between it 

 and the foot-hills of the mountains. 



We find, therefore, that we have at this locality a somewhat narrow 

 belt of the unchanged rocks, packed close together, and inclining at 

 about the same angle, and perfectly conforming with each other, and 

 the metamorphic rocks also. In iDassing up the canon of the little 

 stream from the Boulder Valley we cross the visible edges of creta- 

 ceous formations Nos. 3, 2, and 1, the Jurassic red beds, and the paleo- 

 zoic sandstones, to the metamorphic rocks. While I believe that the ex- 

 tensive series of coal strata all perfectly conform with the older forma- 

 tions, yet as we pass eastward from the Boulder Valley the dip becomes 

 less and less until it ceases in the plains. 



An important question arises as to the cause of the change in the sedi- 

 mentary rocks of this region. That the sandstones forming the huge 

 ridges have been iDartiaEy metamorphosed is clear, though the traces of 

 their sedimentary origin are as plain as ever. 



The limestones of cretaceous formation No. 3 are more compact at 

 this point than I have ever observed them northward; and the coal, 

 along a narrow belt, is far superior to that which is found farther east- 

 ward in the plains. I am inclined to believe that the area from which 

 first-class coal will be obtained in Colorado is very restricted, and will 

 be comprised in a moderately narrow belt along the base of the moun- 

 tains south of Boulder Creek and north of Golden City. 



These changes might be attributed, wholly or in part, to the influences 

 of igneous action in the vicinity. In the vaUey of the Boulder, near 

 Valmont, there is a i^romiuent dike of very compact basalt, which rises 

 up like a wall, but does not seem to have disturbed the tertiary sand- 

 stones in the vicinity. Near Golden City, about twenty miles south- 

 ward, close to the base of the mountains, are two large mesas, or table- 

 lands, covered with a thick layer of basalt, which must have passed up 

 from below in the form of a dike, and flowed over the tertiary rocks. 



These are the only instances of eruptive rocks observed by me from 

 near the South Pass to the Arkansas, a distance of neaj-ly four hun- 

 dred miles. In the Middle Park, just vrest of Loug's Peak, and in the 



