244 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.61. 



The strata of the upper deposits differ greatly from each other in composition, 

 fineness of material, and thickness. At least the whole fossiliferous portion exam- 

 ined has been deposited in water, is tliin-bedded, and very distinctly stratified. It 

 often weathers out in large scales little thicker than ordinary writing paper. During 

 the periods of deposition large numbers of insects and plants were sometimes entombed 

 and preserved in great perfection. While there is a general sameness to the forma- 

 tion in various parts of the basin, yet, upon more particular examination of the 

 strata, great dissimilarity in minor details forces itself upon the attention. Espe- 

 cially is this true when one compares a vertical section in the northwestern basin 

 with one in the southeastern basin. 



The lower part of the formation seems wholly nonfossiliferous, is of a drab color, 

 qmte homogeneous, thick-bedded, and has a decidedly conchoidal fracture. Our 

 limited examination of that portion of the formation left the impression that it was 

 formed by mud flows without much assortment by moving water. This ia offered 

 merely as a suggestion for future work, not as a final conclusion, and its investiga- 

 tion may throw important light on the history of the lake. Near the northwestern 

 end of the basin a shaft has been sunk for some distance into the nonfossiliferous 

 beds, and in other places wells have been sunk, affording excellent sections and 

 showing that the floor deposits extend to considerable depth — just what depth we 

 have not learned. The fossiliferous beds above the floor deposits are approximately 

 20 feet in thickness. 



As regards the geological age of these lake beds there has been 

 some difference of opinion. As they lie on, and are completely sur- 

 rounded by, granite, and further are isolated from all other sedimen- 

 tary rocks by several miles of crystaUines, no aid can be expected 

 from stratigraphy and dependence must be placed entirely in the 

 paleontological contents. As a result of his hasty examination Doctor 

 Peale inclined to regard them as of Pliocene age, but Lesquereux, 

 who studied the plants, at first considered them as probably belong, 

 ing to the upper Miocene. Later, when larger collections became 

 available, Lesquereux came into substantial agreement with Cope, 

 who had studied the scant fish remains, and these beds came to be 

 referred to the lower Miocene or Ohgocene, which is the age usually 

 assigned them in textbooks and elsewhere. Within the past few 

 years, however, there has been a revival of interest in the study of the 

 Florissant flora and fauna, due largely to the activities of Prof. T. 

 D. A. Cockerell,^ and as a result of these studies and comparisons with 

 various floras of this country and Europe, the conviction has been 

 growing that the position originally assigned by Lesquereux is more 

 nearly correct, namely, that the beds are upper Miocene in age. 

 While it is improbable that the final word has been said regarding 

 the exact stratigraphic position of these beds, until conclusive evi- 

 dence to the contrary has been presented the Florissant plant-bearing 

 beds may be regarded as upper Miocene. 



It may be of interest to note in this connection that lake beds of 

 similar lithologic composition, and containing many of the Florissant 

 species of plants, have recently been discovered in other parts of 

 Colorado. These will bo described and discussed in full in a later 

 pubUcation. 



» See Univ. Colorado Studies, vol. 3, No. 3, 1906, pp. 167-176; Amer. Nat., vol. 44, 1910, pp. 31-47. 



