EXPLANATIONS OF PLAINS FORMATION 691 



tions of Lewis and Clark in 1804-6, of Nuttall in 1818, of Long in 1819, 

 of Nicolet in 1839, and of Owen in 1849 brought back many Cretaceous 

 and Tertiary fossils, and for the entire region the general conception of 

 an upraised sea-bottom prevailed. Even so late as the year 1890 Cross® 

 refers to the Table Mountain basalt, near Golden, as long "submerged in 

 the Denver sea/' It is for this reason, doul)tless, more than any other, 

 that the more recent discovery of an abundance of non-marine organic 

 remains at once gave rise to the notion that the extensive terranes of fine 

 loams represented deposits of vast fresh-water lakes rather than of old 

 seas. 



Without exception, the accounts of the so-called fresh-water Tertiaries 

 of the Great Plains seem to be arguments against the theory of their 

 marine deposition rather than a marshalling of facts in support of the 

 possible lake origin which was claimed for these deposits. 



LACUSTRINE ORIGIN 



The first suggestion that the mid and late Tertiary deposits lying east 

 of the Rocky Mountains were laid down in vast fresh-water lakes appears 

 to have been made in 1852 by Dr. John Evans,^ who three years pre- 

 viously had conducted tlie government geologic explorations on the upper 

 Missouri Eiver. In writing of the profusion of organic remains which 

 he everywhere encountered he observes that "all speak of a vast fresh- 

 water deposit of the early Tertiary period." D. D. Owen, under whose 

 official direction these explorations were undertaken and who edited 

 Evans's notes, does not seem to agree with him, but adopts the old view of 

 marine formation. 



Without discussion or setting forth of evidence, and without referring 

 to Evans's published observations, Hayden^ also proposes formally the 

 lacustrine hypothesis, since which time, and with no attempt at critical 

 examination of the fimdamental data, it has been generally accepted. 

 The idea appears to have originated in the circumstance that whereas at 

 the base of the great sequence the beds contain oyster shells and other 

 marine remains, higher up are found some land and fresh-water shells, 

 the bones of many land animals, and remains of plants. In support of 

 a lacustrine origin of the deposits considerable weight is attached to the 

 fact that there is^vast extent and great thickness to the marly silts and 

 fine sands composing the formations. 



The list of writers supporting the lake hypothesis is a long one. Sin- 



•Proc. Colorado Scl. Soc. vol. ill, 1800, p. 124. 



' Geol. Survey of Wisconsin, Iowa, and Minnesota, 18.52, p. 107. 



8 First Ann. Kept. U. S. Geological Survey of the Territories, 1867, p. 58. 



