356 ICEEDINGS OF THE AMEBICAN ACADEMY. 



and no! on the land side of the Bhore 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 ; do mention is 

 made of the gradual encroachment of marginal deposits upon the area 

 earlier occupied by lake waters. Whatever qualifying ideas theauthor 

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

 ami do 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 nol 

 perhaps unnatural, at a time when little attention had been given t'> the 

 importance of subaerial deposits, that geologists should fall into the 

 habit of regarding all Don-marine formations a- lacustrine. To-day 

 there is more reason for critical discrimination, and several extol 

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

 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 dow 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 indications that an unpublished, perhaps unconscious change of opinion 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 \ of 



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

 marine denudation, and substituted therefor the theory of Bubaerial 

 degradation, as advocated l>y Sir A. Geikie; yet conversation with a 

 Dumber <>t' them last year convinced me that such was commonly the 

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

 long( r regarded as exclusively lacustrine 1>\ a certain number of Ameri- 

 can geologists who, although they have published uothing to indicate a 

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

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

 aerial processes can have accomplished in Tertiary time. But the 

 change of opinion cannot be universal, for some of the extract- given 

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

 recent years, and one of the recent monographs of the l'. S Geological 

 Survey Bhows that Borne of our most experienced geologists still follow, 

 the interpretation of earlier years in referring even coarse-textured 



