THE EOCENE OE NORTH AMERICA 469 



mistaken for a lake deposit ; for the safe discrimination of the 

 two classes of deposits must depend on their differences, not on 

 their resemblances. While the marginal sediments of a lake 

 may be coarse, the body of the central sediments^ must be fine 

 and uniform. The marginal parts of a fluviatile deposit may 

 also be coarser than the forward parts, but the latter may be 

 characterized by frequent variations of texture and structure, and 

 occasionally by filled channels and lateral unconformities " (p. 



370- 



Some quotations may be given to show that many descrip- 

 tions of the so-called lake beds would apply equally well to 

 river deposits. Lake terraces are "well marked between Ralston 

 and South Boulder creeks (Colorado), where there is a blending 

 of lake and river terraces. 1 - Here five distinct terraces are trace- 

 able, the lake terraces extending from 100 yards to three miles 

 eastward from the foothills, while those more distinctly of stream 

 origin are from 200 to 700 feet in width."* Here the lake and 

 river terraces are not clearly distinguished : the width of the 

 terrace seems to be the principal criterion, and the limits assigned 

 to lake and to river terraces overlap. According to the figures 

 given, the river terraces here reach a width of 700 feet, while 

 some of the lake terraces are only 300 feet wide. Again, from 

 the same monograph, with reference to the present inclination 

 of both Tertiary and Pleistocene deposits, it is said that there 

 is an inclination "in round numbers of ten feet to the mile from 

 the foothill region to the valleys of the Missouri and Missis- 

 sippi ; " this would not admit of the holding of lake waters." 3 



King's Report of the Survey of the 40th Parallel abounds in 

 descriptions of so-called lake beds like the following: "Rough, 

 gritty conglomerates, passing up into finer-grained sandstones, 

 and at certain points developing creamy, calcareous beds" (p. 

 405). The most characteristic exhibition is in the basin of Ver- 

 million Creek, where a fuller section is displayed. It is made up 



'The italics are mine. 



* Emmons: Geol. of Denver Basin, Monograph XXVII. U. S. Geol. Surv., p. 9, 

 1896. 



3 Ibid., p. 40. 



