]56 GEOLOGY OF THE DENVER BASIX. 



which is not derived from the crystalline rocks of the mountains, though 

 a portion comes through the medium of older sedimentary formations. 

 The Denver sandstones and conglomerates, on the other hand, consist 

 chiefly of debris of eruptive rocks of the andesite family, with which 

 material of other origin is prominently associated only in the upper hori- 

 zons. For several hundred feet at the base of the scries the strata contain 

 almost nothing derived from granite or gneiss. Upward in the series the 

 latter material reappears and finally predominates very largely, although 

 no horizon is free from andesitic debris. 



Historical. — Before the identification and description of the Denver forma- 

 tion by the writer in 1888, the strata in question had been uniformly treated 

 as a part of the Laramie. The strata of Table Mountain — the most typical 

 of the series — had been examined and described by various geologists, and, 

 as the matrix for well-preserved plant remains, were extensively repre- 

 sented in large collections of the country. The strata have been referred 

 to as simple " sandstones" or "clays," and the material composing the sand- 

 stones seems in no instance to have attracted attention, though conglomeritic 

 layers everywhere alternate with simple sand and clay strata. 



As far as the writer is aware, the first geologist to study strata belong- 

 ing to the Denver beds was J. L. Le Conte, 1 who examined the vicinity of 

 Golden in 1867. He refers to Table Mountain as follows: 



East of Golden City is a plateau nearly 600 feet high, composed of horizontal 

 sandy and clayey strata, containing badly preserved vegetable remains; it is capped 

 by :i thick sheet of basalt, which has been poured out from craters, the remnants of 

 which exist on the grassy plains of the mesa as elliptical ponds. 



F. V. Havdetr personally visited South Table Mountain in 18(>9, and 

 in his report speaks of the mesas and of the sandstones and clays bearing 

 plant remains, classifying the latter as belonging to the "Lignitic group." 

 Green Mountain is spoken of as "entirely composed of the coal strata." 



In a later publication 3 Havden repeats the statement that "under the 

 basaltic caps of Table Mountain there is a great thickness of Lignitic beds." 



'Notes on the Geology of the Survey for the Extension of the Union Pacific Railway from 

 Smoky Hill River, Kansas, to the Rio Grande. Philadelphia, 8°, 1868, p. 76. 



-' Preliminary Field Report of the 1 T . S. Geol. Survey of Colorado and New Mexico, 1869, p. 35. 

 :i U. S. Geol. Survey of the Territories, Bulletin 4, second series, p. -15. 



