TF. Cross- — Post- Laramie Deposits of Colorado. 19 



Aet. III. — Post- Laramie Deposits of Colorado; by Whit- 

 man Ceoss. 



[Published by the permission of the Director of the U. S. Geological Survey.] 



Introductory. 



Among questions in American geology which have given 

 rise to prolonged discussion and controversy, few if any have 

 been more prominent than that as to the age of the great 

 Laramie Formation or Group of the Rocky Mountain region. 

 This observation, made by Mr. Clarence King in the final 

 report of the Fortieth Parallel survey, in 1878, is still more 

 true to-day than it was at that time. When it is considered, 

 however, that a very large part of the earlier publications were 

 based upon mere geological reconnaissance, and that the area 

 involved extends across the United States in its most inacces- 

 sible and least known portion, from Mexico to British 

 America, it must be plain that controversies and misunder- 

 standings were unavoidable, and indeed quite natural. 



A few years ago it seemed to many geologists that the Lar- 

 amie question was practically settled, or in a fair way to 

 settlement. But, as certain areas of the West have been more 

 carefully explored, the question has been in a measure re- 

 opened, but with a change in its phase, so that it is not now so 

 much " To what age does the Laramie belong ?" as it is 

 " What belongs to the Laramie ?" For the newer researches, 

 whatever their direction, all tend to show that the Laramie 

 has been a great omnibus division into which has been cast 

 everything ascertained to lie between the marine Cretaceous 

 and the lowest recognized Eocene deposits, together with a 

 number of other formations whose positions were not deter- 

 mined. It is the aim of this paper to show that there exists at 

 least one important group of formations which have been 

 considered as belonging to the Laramie, but which are very 

 markedly distinct from the formation to which that name 

 properly belongs. 



As far as the writer is aware the first definite proof that a 

 given section assigned to the typical Laramie contained mem- 

 bers separated by important unconformities and of widely 

 different lithologieal character, was afforded by the work done 

 in the vicinity of Denver, by the Colorado Division of the U. 

 S. Geological Survey, in charge of Mr. S. F. Emmons. A 

 preliminary account of these researches was presented to the 

 Colorado Scientific Society, July 2, 1888, in articles by G. H. 



