Perry.] 76 [February 28, - 
We should, first of all, remember that in one view of the matter, 
what may be perhaps properly called an Aionian summer, — a great 
summer of the ages —was just drawing to its close. A previous 
warm season of the ages had long before passed its meridian, and, as 
its decline advanced, its mild evening was perhaps only the precursor 
of the long period of cold which was to follow. So the fact sus- 
gested by Prof. Agassiz,’ that voleanic agency was very active during 
the last part of the Tertiary era —a fact which I can not stop to 
account for and explain in this paper — supplies an important con- 
sideration in this direction. Be the cause of this outbreak what it 
may, the effect upon the climate must have been considerable. 
Again, there is the supposition which astronomers have from time to 
made, that the sun is a variable body, liable to be much warmer at 
given periods than at others, from the falling into it of planetary 
masses; that at about the epoch in question there was a large addi- 
tion of this kind to its fuel; and that there thus resulted a great 
increase in the degree of heat which it sent forth. While this sup- 
position is not perhaps improbable, while it is indeed rather plausi- 
ble, it is still only a conjecture, at best a guess, though possibly one 
of prime importance, which we as yet have no recognized means of 
either proving or disproving to our satisfaction. Admitting that such 
may have been the fact, I am indisposed to press it; meanwhile 
there is the point already mentioned to which the rocks themselves 
bear witness; volcanic agency was peculiarly intense in the times 
just preceding the glacial. 
But this agency, and most of the facts thus far presented, are 
mainly indicative of warmth. The glaciation is not in this way 
accounted for. In order to the production of the immense masses 
of ice supposed to have existed, there must have been a far greater 
than ordinary degree of cold. The high temperature which had 
prevailed would naturally fall to some extent, with the diminution of 
the Plutonic agency which caused it. This, however, would be only 
the restoration, or a tendency toward the restoration of the equilibrium 
of the prevailing temperature of the earth. Far more cold than 
this was probably requisite to produce such results as fol- 
lowed; far more than any mere medium, or average of temper- 
ature, estimated according to the usually recognized data. And for 
so marked a change is there any adequate cause? A hint at this has 
1In a Lowell Institute lecture. 
