164 
GEOLOGY: E. BLACKW ELDER 
not have such anomalies as the actual interbedding of strata in Montana 
containing marine invertebrates assigned to the Cretaceous with those con- 
taining land plants identified as Eocene; or as the Triassic in Idaho resting 
conformably on rocks containing only Pennsylvanian fossils. Many other 
cases of similar perplexity will occur to those who have had much to do with 
age questions in stratigraphy. All things considered, it seems to me that the 
improvement of correlation methods and a more general acceptance of these 
improvements is at the present time the thing most to be desired by the 
earth historian, for nowadays it is one of the chief causes of friction among us. 
What service can the study of the sediments and sedimentary rocks render 
in connection with these two problems? First it offers the best and most 
comprehensive means of working out the history of climate. We already 
understand rather fully the climatic significance of such deposits as beds of 
coral limestone, of tillite, and of saline formations. We have probably reached 
a similar comprehension of the red beds, loess, and certain othei 4 types, although 
we are not yet fully agreed among ourselves regarding them. Before long we 
may expect to know as fully the climatic significance of the coal-bearing gray 
sediments, and eventually even of most of the marine deposits. For even 
among the, latter no matter how great the importance of the work of bacteria, 
algae, foraminifera, and other organisms is, it becomes increasingly evident 
that the very activities and processes of these organisms are largely dominated 
by climate, either directly or indirectly, and that they are forced to make a 
record of climatic changes in the marine sediments to which they contribute. 
For example, the prevalence of siliceous in place of calcareous ooze in the 
Antarctic Ocean is probably due to biochemical factors that depend on climate. 
The study of climatic history is not only necessary for its own sake as a 
division of the larger earth history, but it has an important bearing upon the 
attainment of the other desideratum, namely, more reliable correlations,. 
Climatic changes are widespread in their influence. Some, like the cooling 
off during the last glacial period, seem to have affected the whole earth. They 
influence both land and sea deposits, and hence leave their impress on all 
sedimentary formations. In comparison with the slow progress of geologic 
events, the effects of climatic change are felt quickly. We seem justified in 
believing that altered climates do not ordinarily migrate slowly from region 
to region, and on this assumption it may generally be presumed that the 
results are essentially simultaneous over large areas. For these reasons cli- 
matic changes should serve as very delicate indicators of time relations. They 
are likely to be especially valuable because their record is most clear in the 
terrestrial sediments, where fossils — our customary reliance — are apt to be 
rare or absent. 
From the earliest days of geology attempts have been made to correlate 
strata in different places by means of lithologic character. Many of these 
attempts have met with either failure or only partial success. In compara- 
tively recent years, however, more refined methods have yielded much better 
