LAKE BEDS. 71 



The red sandstones of Mount Silverheels, above the beds assumed to 

 be Upper Coal Measures in this report, are noticeable for their coarse grain 

 and for the abundant pebbles of Archean rocks which they contain. In 

 some intercalated shaly beds just east of Fairplay, Professor Lakes found 

 plant remains and fossil insects. The former were determined by Professor 

 Lesquereux to be undoubtedly Permian and the latter by Mr. A. Hyatt to 

 be as certainly of Triassic age. In such conflict of evidence it seems safer 

 to trust to that of animal life, since it is already well established that in 

 America plants came into existence in Cretaceous time which in Europe 

 have always been considered to have made their first appearance during 

 the Tertiary. 



QUATERNARY. 



The Quaternary formations which have been designated by special 

 colors on the maps and sections are the Glacial or Lake beds, and the 

 Post-Glacial or recent detrital formations. As already shown, there is 

 evidence of the existence, during the intermediate flood period of the 

 Glacial epoch, of a large fresh-water lake at the head of the Arkansas 

 Valley, in whose bed was deposited a considerable thickness of coarse and 

 rudely-stratified beds of detrital material from the adjoining mountains. 



Glacial or Lake beds (q). Owing to the limited opportunities afforded for 

 obsei'ving these beds in place, it was impossible to obtain a complete sec- 

 tion of them or an accurate estimate of their aggregate thickness. The 

 maximum thickness observed is about 300 feet; their material is generally 

 coarse, and, as might be expected, very much coarser along what is known 

 to have been the shore line of the lake. The finest of the beds consist of a 

 calcareous marl, whose development seems to have been extremely local. 

 The prevailing beds are a loose friable sandstone, resembling granite decom- 

 posed in place, consisting largely of grains of quartz and feldspar, and 

 often somewhat iron-stained. These beds frequently alternate with those 

 of coarser material, which form a rude conglomerate. The coarser beds 

 contain both angular fragments and bowlders of the rocks which make up 

 the range, and lithologically can hardly be distinguished from the Wash of 

 the succeeding formation; but, where any considerable thickness of the 



