STRATIGRAPHIC CLASSIFICATION 501 



But it would be a slur upon paleontology to consider such 

 arrangement final. Even were the dividing planes between 

 great systems fixed, the precise recognition might require 

 detailed investigation of faunas ; but in many cases they are not 

 fixed beyond question, either by faunas or b}' unconformities. 

 Geology has not arrived at a final classification. We need still 

 to accumulate physical and biological facts, to keep them dis- 

 tinct, and to compare them from district to district, and from 

 country to country, before our systems can be said to be estab- 

 lished. 



The writer is indebted to his colleague, Mr. Whitman Cross, 

 for a case in point — that of the Hermosa, Dolores, and Rico, 

 so-called formations. Quoting from Mr. Cross's statement 

 before the Geological Society of Washington (as revised by him), 

 the case is as follows : 



[A diagram exhibited] represented the problems in subdivision of the 

 great series of alternating shales, sandstones, conglomerates, limestones, and 

 strata of intermediate lithologic character represented in the Rico quad- 

 rangle, southwestern Colorado. This series of rocks, about 4,500 feet in 



"5 



S Dolores formation (Triassic) Red 



Rico formation (Pernio Carboniferous) 



"re Hermosa formation (Upper Carboniferous) Gray 



thickness, extends from the base of the Upper Carboniferous to the top of 

 the Trias. The lower 2,000 feet of strata are characterized by Upper Car- 

 boniferous fossils. The intermediate three or four hundred feet by a well- 

 defined Permo-Carboniferous fauna, and a considerable portion of the upper 

 2,000 feet by Triassic fossils. 



Recognizing the importance of the time divisions indicated, the Upper 

 Carboniferous strata have been grouped as the Hermosa formation ; the 

 strata bearing the Permo-Carboniferous fossils as the Rico formation ; the 

 strata containmg Triassic fossils as the Dolores formation. With the Tri- 

 assic strata are provisionally included at the present time other strata not 

 known to be fossiliferous. The upper limit of the Dolores is a definite litho- 

 logic and structural horizon. The lower limit cannot be determined upon 

 lithologic grounds alone. As a line micst be drawn on arbitrary grounds, the 

 Dolores complex has been extended below to the uppermost stratum containing 



