66 GEOLOGY OF OHIO. 
Greenland, Spitzbergen, etc., the remains of more than 200 species of 
plants have been procured, which show that all the arctic land then en- 
joyed a mild climate, and were covered with luxuriant forests, such as 
would not now grow on this continent north of the 40th degree of lati- 
tude. We have also abundant proof that in the next succeeding age, the 
Quaternary, an extreme arctic climate prevailed over most of North 
America, and great ice-sheets, if not continental glaciers, reached as far 
south as Cincinnati; subsequently the climate ameliorated, and the gla- 
ciers retired to Greenland. Very naturally, when incontestible proof of 
these changes had been gathered, much discussion was excited in regard 
to their probable causes. This discussion is still going on, and there is 
great diversity of opinion on this subject, even among our most learned 
and wisest geologists. For the solution of the problem, the most detailed 
and laborious investigations have been made, such as required the pro- 
foundest knowledge of mathematics, physics, astronomy, and geology. It 
is evident, therefore, that anything like a thorough review of the subject 
would be out of place in this report of the facts gathered by the Geologi- 
eal Corps, and the briefest possible sketch of the present attitude of the 
question is all I shall attempt here. So much as that seems, however, to 
be required for the gratification of the interest which the facts I have | 
given may have excited. 
Various suggestions have been made to account for the ice period, which 
are mere efforts of the imagination; such as the passage of the solar 
system through cold spaces in the universe, changes in the position of 
the axis of the earth, etc. These, however, have been generally rejected, 
as they are destitute not only of all proof, but of probability. Sir Charles 
Lyell, perhaps the most sagacious and conservative of modern geologists, 
has attempted to explain the alternations of climate to which I have 
alluded, by referring them to changes in the distribution of land and 
water; arguing that by the concentration of land about the pole, where 
snow and ice could gather in unlimited quantities, and propagate the 
cold of which they themselves were the product, arctic conditions might 
be brought down as low as during the glacial period. He also claims 
that when the opposite condition prevailed in the distribution of land 
and water, and the continents were spread out under the tropical sun to 
absorb and disseminate its heat, while an open sea occupied the arctic 
regions, glaciers could have no existence in either hemisphere. This 
view has been quite generally accepted by geologists, and is that adyo- 
‘cated by our highest American authority in physical geology, Prof. Dana. 
The conviction, however, has, of late years, been gaining ground, that 
this theory was inadequate to account for the stupendous changes of 
