GEOLOGY: E. BLACKW ELDER 
165 
results. For example, Rogers and Stone in their recent study of the Lebo 
shale of Montana have shown that it can be identified over large areas by its 
andesitic particles, which record eruptions of volcanos farther west at a single 
epoch in Cretaceous times. 
In the future it will undoubtedly be possible to make far greater use of the 
physical characteristics of sedimentary rocks in correlation — not so much by a 
direct matching of similar rocks as by an indirect process of first elaborating 
from the rocks the climatic and physiographic conditions and their changes in 
time, and then correlating these. It should in fact be as feasible to correlate 
by means of climatic history as to correlate by diastrophic history. Indeed 
it is already beginning to be the practice of the most progressive stratigraphers 
to make their correlations not simply on the basis of faunas nor on the basis 
of diastrophism, but on the compound basis of life, climate, topography, vul- 
canism, and diastrophism with due regard to their mutual relations and 
dependences and their relative values. It is my own expectation that this 
practice will soon become general. 
What is Needed. — We may reasonably hope to understand eventually all 
of the sedimentary rocks at least as well as we now know any; but at present 
our knowledge is very uneven, being tolerably complete for some types and 
very slender for others. Among the sediments which are as yet but partly 
understood the following may be mentioned by way of illustration of our 
needs: limestone conglomerates and oolites, sedimentary iron ores, chert, 
jasper, etc., gray flags and shales with lean faunas, dolomites, phosphorites, 
greensands, black oily shales, lithographic limestones, and rhythmically 
alternating shale and limestone. Of course, there are many others. 
To advance more rapidly this part of the scientific battle front, several 
things are needed. Among the most useful are the prolonged and intensive 
studies of certain modern types of deposits and the processes of their deposi- 
tion, as illustrated by the investigations that have been carried on recently 
by T. W. Vaughan and his associates in the Florida region, as well as by the 
careful study of mountain stream work by G. K. Gilbert in California. Some 
of these problems are too large to be attacked by most individuals, but require 
for their successful execution the aid of our strongest scientific institutions and 
the cooperation of a number of investigators over a period of years. 
Hardly less valuable are the close and detailed studies of ancient sedimen- 
tary rocks, such as Barrell's interpretation of the Mauch Chunk shale, the study 
of the western Red Beds by C. W. Tomlinson, and of the dolomites by half a 
dozen or more geologists in the last few years. Probably this method must be 
our sole reliance in the case of certain peculiar sediments which, so far as known, 
are not being formed on the earth at the present time. These are illustrated 
by the thick, rich beds of phosphorite in Idaho and perhaps by the stratified 
iron ores of eastern Brazil. 
Our understanding can be advanced in a most helpful way also by careful 
experimentation, such as that on sun-cracks reported by E. M. Kindle, or the 
