4 CHARLES RICHARD VAN HISE 



are scarcely less great. Only recently has serious study been under- 

 taken to discriminate subaerial and subaqueous deposits. This 

 subject will not be gone into here, since it is one which has been 

 recently discussed in several extended papers. I may, however, 

 speak of one point. So far as we can yet determine the subaerial 

 deposits are in general not so well assorted nor so likely to be sharply 

 separated into distinct formations as the subaqueous deposits. This 

 statement is believed to hold although it appears that under exception- 

 ally favorable conditions the aerial deposits may be pure quartzose 

 sands. Consequently cleanly assorted quartzose sands, pure hme- 

 stones, and series composed of sharply contrasted formations are 

 regarded as strongly favoring the idea of subaqueous deposition. 

 As yet there is no evidence that air has the discriminating capacity 

 which water has in producing cleanly assorted sands. If it is difficult 

 to discriminate subaerial or subaqueous deposits, it is much more 

 difficult to discriminate subaqueous deposits of the inland lakes and 

 seas from those of the ocean. 



6. Unconformities. — Unconformities are of great assistance in 

 classification and correlation. It has been intimated that the great 

 physical movements producing unconformities are frequently the 

 real causes of faunal changes. Irving was the first fully to realize 

 the importance of unconformities in correlation. The criteria by 

 which unconformities are determined and their magnitude and 

 significance analyzed cannot be discussed in a short paper. Those 

 interested in this aspect of the subject must be referred to the original 

 discussions.^ 



It should be remarked, however, that unconformities may have 

 a very variable extent and significance. It is now realized that a 

 sharp orogenic movement may take place resulting in uplift, erosion, 

 subsidence, and therefore discordance of strata, which may not affect 

 an adjacent area. Thus it should clearly be understood that it cannot 

 be assumed that unconformities due to orogenic movements are more 

 than of district extent. There are, however, great movements of 

 uplift and subsidence which are continental and may be even inter- 



I Roland Duer Irving, "On the Classification of the Early Cambrian and Pre- 

 Cambrian Formations," Seventh Annual Report, U. S. G. S., pp. 365-454; Charles 

 Richard Van Hise, " Principles of North American Pre-Cambrian Geology," Sixteenth 

 Annual Report, V. S. G. S., pp. 724-34. 



