308 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
Of the ‘‘critical periods” at the close of the Paleozoic, Mesozoic, 
and Cenozoic eras, we know that the first and last were accompanied 
by glacial Slemuties but the Mesozoic, though a time of very extensive 
mountain making aud great and soelgnged volcanic activity in North 
America, did not close with a glacial, but only with a slightly cooled 
climate. Not only this, but we find that voleanism was renewed in 
the Cordilleras of North America throughout much of the Eocene, and 
yet there was developed no glacial climate at this time. In the same 
way the marked temperature reduction at the close of the Cenozoic 
in the Pleistocene was subsequent to the Miocene and Pliocene move- 
ments of this period and not comcident with them, while that of the 
Paleozoic appears to fall in with the rise of the Urals and Appala- 
chians, though but little volcanism seems to have accompanied the 
movements in North America. It should also be said that equally 
extensive movements were going on in Europe in the rise of the 
Kuropean Alps during the geologic times before and after the Permic 
glaciation, and that the earlier movements did not appreciably affect 
the climate. 
Again, there was decided mountain making toward the close of the 
Siluric in the formation of the Caledonia Mountains all along western 
Europe from Spitzbergen to Scotland, with marked volcanic extru- 
sions during the Siluric and early Devonic in Maine, the Maritime 
Provinces of Canada, and Europe. Yet we have no glacial climate 
at these times, certainly not in the Northern Hemisphere; rather it 
seems that the temperature was mild the world over. It is possible, 
however, that the Table Mountain tillites of South Africa may coin- 
cide with this time, and if so a colder temperature affected the 
Southern Hemisphere only locally. 
On the other hand, the “life thermometer” indicates a cooled 
period at the close of the Triassic and the following Liassic, but this 
reduction of temperature, again, is geologically subsequent to, rather 
than coincident with the marked volcanic activity of the Triassic in 
many widely separated places. 
Finally, there were earth movements of considerable magnitude at 
the close of the Lower Cambric, Ordovicic, and Jurassic that were 
not accompanied by glacial climates. At all of these times there 
appears, however, to have been a drop in temperature, slight for the 
two first-mentioned periods and more marked for the third one, for 
here we find in the austral region, during earliest Cretacic times, win- 
ters alternating with summers. 
We may therefore conclude that volcanic dust in the isothermal 
region of the earth does not appear to be a primary factor in bringing 
on glacial climates. On the other hand, it can not be denied that such 
periodically formed blankets against the sun’s radiation may have 
assisted in cooling the climates during some of the periods when the 
continents were highly emergent. 
