SOME FUNDAMENTAL CONCEPTS OF EARTH HISTORY 165 
speculations which had preceded it. Some of the facts of 
astronomy and physics which recent research has marshalled 
against the theory have been stated above. It may be added here 
that the Nebular Hypothesis provided an immense atmosphere 
during the early stages of the earth’s evolution with gradual 
diminution until presumably its rarity would allow the total 
drying up and freezing of the earth. As it has been expressed, 
“Our recent icy stage was but an October frost; December was 
yet to come.” But recent studies have shown the presence of 
glacial epochs almost from the beginnings of known geologic his- 
tory as written in the stratified rocks. Furthermore, evidences 
of dry periods far back in the past have come to light and have 
still further disturbed the regularity of the supposed course of 
events. Again, the granitic masses which were once supposed 
to represent the very rock foundations of the earth’s crust 
have proved to be later intrusions and not the original crust at 
all. The globe itself seems to be adding its testimony to the 
insufficiency of the old theory of its origin. 
Some, years since, while Dr. T. C. Chamberlin was engaged 
in a study of the glacial deposits of Wisconsin, of which state 
he was State Geologist, he became interested in an investigation 
of the causes of glacial periods. This led him gradually back- 
ward to the broader theme of the origin of the earth and the 
sufficiency of the Laplacian Hypothesis. After he became presi- 
dent of the Univeristy of Wisconsin and since he has been head 
of the department of geology at the University of Chicago he con- 
tinued his researches, with the cooperation of Dr. F. R. Moulton, 
the able astronomer and mathematician. The discrepancies which 
were discovered as a result of their computations and which 
have been outlined above weakened their faith in the old^r view 
and after several attempts to patch it up or to use some other 
existing hypotheses, such as the meteoritic of Lockyer and of 
Darwin, they found it necessary to set about the more difficult 
constructive task of formulating a new hypothesis which would 
avoid the pitfalls that had wrecked the old one and which would 
fit observed facts and demonstrated laws. Their progressive 
results were subjected constantly to the most rigorous mathe- 
matical scrutiny and the completed hypothesis — the Planetesimal 
Hypothesis — seems to meet the most exacting demands of modern 
science. A brief outline of this hypothesis must suffice here. 
