TERTIARY LAKE BASIN OF FLORISSANT 149 



scale being grossly exaggerated in proportion to the horizontal scale, 

 in order to appeal more readily to the eye. 



That the time of the existence of this lake was one of great volcanic 

 activity in the region is quite evident. This conclusion does not depend 

 merely upon the stratigraphic relations of volcanic rocks to the lake 

 beds, but more particularly upon the character of the sedimentary 

 deposits themselves. These beds are composed chiefly of volcanic 

 ashes, mud and sand, the component particles of which are generally 

 somewhat, though not very much, worn by the action of the water. 

 The conclusion reached in both field and laboratory is that the deposits 

 were formed largely by volcanic ashes from repeated eruptions fall- 

 ing upon the surface of the water and settling to the bottom, assorted 

 by the sluggish lake currents; also by volcanic mud and ashes falhng 

 or flowing into positions where they were rapidly washed into the lake 

 by rains, streams, and waves without very much grinding. 



The strata of the upper deposits differ greatly from each other 

 in composition, fineness of material, and thickness. At least the whole 

 fossiliferous portion examined has been deposited in water, is thin- 

 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 formation in various parts of the basin, yet, upon 

 more particular examination of the strata, great dissimilarity in minor 

 details forces itself upon the attention. Especially 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 non-fossihferous, 

 is of a drab color, quite homogeneous, thick-bedded, and has a decid- 

 edly 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 is offered merely 

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

 tigation may throw important Hght on the history of the lake. Near 

 the northwestern end of the basin a shaft has been sunk for some 



