356 PROCEEDINGS OF THE AMERICAN ACADEMY. 



side and not on the land side of the shore line, below the water level 

 and not above it. Not even the coarse conglomerates are described as 

 the superaqueous or subaerial parts of marginal deltas ; no mention is 

 made of the gradual encroachment of marginal deposits upon the area 

 eailier occupied by lake waters. Whatever qualifying ideas the author 

 may have held on these points, they do not appear in his descriptions, 

 and no student would gather them from his text. Yet it is rather in 

 explicitness and detail of statement than in interpretation that this 

 report differs from many others. 



6. The Arapahoe and Denver Formations of Colorado. — It was not 

 I^erhaps unnatural, at a time when little attention had been given to the 

 importance of subaerial deposits, that geologists should fall into the 

 habit of regarding all non-marine formations as lacustrine. To-day 

 there is more reason for critical discrimination, and several exti'acts 

 given further on will show that some geologists have been led to new 

 interpretations of the origin of certain western fresh water Tertiary 

 formations. It is not improbable that other than a lacustrine origin 

 would be attributed to many of these formations, or to many parts of 

 them, if they were now seen for the first time. There are indeed some 

 indications that an unpublished, perhaps unconscious change of opinion 

 has to some extent taken place on this subject, similar to that by which 

 many of the geologists of Great Britain have been transformed from sup- 

 porters of the theory of marine abrasion to that of subaerial degradation 

 in the production of peneplains. No one would suppose by reading 

 British geological essays of recent years that the British geologists of 

 to-day had very largely given up Ramsay's theory concerning plains of 

 marine denudation, and substitiited therefor the theory of subaerial 

 degradation, as advocated by Sir A. Geikie; yet conversation with a 

 number of them last year convinced me that such was commonly the 

 case. Similarly, it may be that our western Tertiary deposits are no 

 longer regarded as exclusively lacustrine by a certain number of Ameri- 

 can geologists who, although they have published nothing to indicate a 

 change of opinion, may have come by more or less unconscious revision 

 of theories to recognize the great accumulative work that various sub- 

 aerial processes can have accomplished in Tertiary time. But the 

 change of opinion cannot be universal, for some of the extracts given 

 above from the accounts of our western Tertiaries are from reports of 

 recent years, and one of the recent monographs of the U. S. Geological 

 Survey shows that some of our most experienced geologists still follow 

 the interpretation of earlier years in referring even coarse-textured 



