GEOLOGY: E. B LAC KW ELDER 
163 
chromosomes, is confined to the male line and it is possible that its loss of 
function is due to a lack of variable reaction. It experiences in effect, a most 
intensive form of inbreeding and shows the characteristic results of such un- 
varying reactions. For it, there is no opportunity to eliminate greater or less 
variables, as is the case with the x-element during maturation in the female. 
The ultimate problem is, of course, to determine why such a difference between 
homologous elements should exist. 
THE STUDY OF THE SEDIMENTS AS AN AID TO THE EARTH 
HISTORIAN 
By Eliot Blackwelder 
Department of Geology, University of Illinois 
Communicated by J. M. Clarke, March 29, 1918 
Objectives of the Earth Historian. — We are, of course, still immensely far 
from our ultimate goal, which is a complete understanding of all the past 
states and events of the earth, or as Professor Salisbury used to put it, "the 
complete geographies of all past epochs." Progress toward this unreachable 
goal will be most favored if the advance is made rather uniformly, all along 
the front. It is true that such progress is often made by pushing out salients, 
but the further extension of such salients is usually impossible without cor- 
responding support from the flanks. 
In the past we have gone ahead much farther along certain lines in geologic 
history than along others. The history of life and of faunal succession has 
been cultivated assiduously for generations and is, on the whole, much better 
understood than other phases of the subject. Although not so well known in 
detail, the history of diastrophism is now fairly well blocked out and the mere 
continuation of studies already under way is likely to afford us in the near 
future a serviceable understanding of the sequence of major earth movements. 
The most backward points in the general advance just now are in two sectors: 
That of the history of climate, and that of the principles of chronology and 
correlation. 
The importance of climate arises from the fact that it is one of the most 
powerful factors, if not indeed the dominant factor, controlling not only the 
sculpture of the land but the nature of the deposits that are made both on land 
and in the sea. 
Secondly, the principles of correlation must be understood better than they 
are now, before we can bring into their proper time relations the various events 
and conditions of which the sediments give us record. In spite of the im- 
pressions in elementary text books of geology, I think it will be generally 
admitted, by those who have carefully considered the question, that we do not 
yet know these principles with satisfactory accuracy. If we did, we should 
