MOVEMENTS RECOGNIZED BY SEVENSON AND PEALE. 249 



representation of the Upper Silurian and Devonian both in fossils aud in 

 strata, he finds no evidence to prove that the region was lifted above water 

 daring these times, but considers that the general movement of the land dur- 

 ing Palaeozoic time was a subsidence, and that where Carboniferous lime- 

 stone rests directly upon the Archaean there were islands in the early Palaeo- 

 zoic seas which became submerged in Carboniferous time. 



J. J. Stevenson, in the course of his explorations in Colorado and New 

 Mexico in 1873, noted several unconformities and drew the following con- 

 clusions: 



"The Rocky Mountain system, therefore, is the result of four especially marked 

 upheavals, the first, at the close of the Carboniferous ; the second, at the close of the 

 Trias ; the third, at the close of the Cretaceous, and the fourth, during the Tertiary. 

 Of these, the first and the third were the most general in their effect." 



He also recognized the unconformity of overlying beds with the Archaean. 

 In his subsequent more detailed work in southern Colorado and northern 

 New Mexico he does not seem to have found reason to modify these general 

 conclusions. 



Hayden Survey. — The beautiful geological atlas of Colorado,* showing the 

 result of the combined labors of the various members of the Hayden Survey, 

 furnishes a most valuable record of the geology of the Rocky Mountain region. 

 Unfortunately no systematic discussion of their field observations has yet been 

 made to present the final orographical conclusions which would be drawn with 

 the consensus of all who were engaged in the work. In the absence of such 

 a discussion inferences must necessarily be drawn from the graphic repre- 

 sentation of facts given by the maps, where personal verification in the field 

 has not been possible. Such verifications as have been made have proved 

 the substantial accuracy of the geological outlines laid down on these maps, 

 except in southeastern Colorado and in the San Juan mountains, where at 

 the various points examined the facts of nature show such wide divergence 

 from these outlines, as laid down by Mr. Endlich, as to throw serious dis- 

 credit upon all of his field work. 



Dr. A. C. Peale, of this Survey, has since summarized the results of his 

 own observations in Colorado as follows : f 



" 1st. In very early times in Colorado there was Archaean land rising above the 

 Palaeozoic sea. As the Carboniferous age progressed this land diminished by en- 

 croachment of the sea, due to subsidence of the land. This subsidence continued 

 through Triassic, Jurassic, and Cretaceous time into the early Tertiary. 



: '2nd. At the close of the Lignitic, there was a physical break followed by subsi- 

 dence (at least locally), and subsequently by elevation after the deposition of the Mio- 

 cene strata. 



* Field work 1873, 1874. 1S75, 1870. 



tJAmer. Jour. Sci., 3d Ser., Vol. XIII, Mar. 1877, p. 181. 



