100 CHARLES RICHARD VAN HISE 
5. Subaerial or subaqueous deposits —Closely connected with the 
third and fourth criteria is the question as to whether the deposits were 
laid down under air or under water. It is clear that the conditions 
of deposition of these two classes of rocks are so different and the 
nature of the formations which may be contemporaneous so variable, 
that there is great difficulty in correlating the two. Also it is plain 
that the difficulties in correlating disconnected continental deposits 
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 lime- 
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 
tRoland 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, U.S. G.S., pp. 724-34. 
