DAVIS. ROCKY MOUNTAIN TERTIARIES. 357 



fresh-water deposits to lakes, without any published discussion of other 

 possibilities. 



The following extracts from the monograph on the " Geology of the 

 Denver Basin in Colorado," above referred to, gives a fair idea of the 

 emphasis that there is placed upon the lacustrine origin of the Arapahoe 

 and Denver formations ; all the page references here made being to 

 Monograph XXVII. of the U. S. Geological Survey. "After an 

 erosion of the Laramie beds ... a considerable fresh-water lake 

 [Arapahoe] was formed and sedimentation again set in. What the 

 exact area of this lake was it is not possible now to determine; its 

 extent was undoubtedly considerably larger than that covered by its 

 'beds at the present day, especially to the northward" (31). "The 

 movement which caused the drainage of the [Arapahoe] lake . . . 

 was succeeded after a considerable lapse of time by a depression sufficient 

 to allow of the formation of a second [Denver] lake. . . . The nature 

 of the depression which produced such lakes without admitting marine 

 waters ... is not readily conceivable" (32). "The beds deposited 

 in the Denver lake reached a thickness of over 1,400 feet along the 

 Hanks of the mountains, but were probably somewhat thinner toward 

 the middle of the basin " (33). Certain deposits are referred to as hav- 

 ing been laid down near the shore line, but always as if on the lake 

 side of it; for example, "some exposures of Denver strata which clearly 

 show the immediate proximity of the old shore-line" (183); ". . . 

 materials might have been derived from the eastern shores" (201). 

 Several contemporaneous lava flows are said to have been "poured out 

 upon the surface of the sea-bottom " (34). One of these is " the basaltic 

 sheet of Table mountain which was poured out upon the floor of the 

 shallow Denver sea" (1G1 ; see also 291, 292) ; not that direct proof is 

 given of the presence of standing water into which the flows advanced, 

 but that the presence of standing water i$ involved in the theory of the 

 lacustrine origin of the underlying strata, even though some of them are 

 conglomerates. One of the most significant observations bearing on the 

 conditions of deposition is likely to pass unnoticed by many readers, because 

 it is given only an inconspicuous place in the description of details : — 

 " The preseuce of considerable tree stumps in erect position with roots in 

 mud layers and broken trunks in sand or gravel, shows that the water 

 was shallow or even that low-land masses alternated with shallow seas. 

 Probably the latter was the case " (1G8). Nevertheless, the alternations 

 of low-land and shallow water here suggested are elsewhere unmentioned, 

 the usual terms for the area of deposition being " the sea," " the lake." 



