436 Climate of the Glacial Period. (October, 
Mr. Croll has pointed out that though we have no evidence 
to support his theory in the successive faunas and floras of 
the Tertiary strata, yet in the Eocene beds of Switzerland 
and the Miocene of the north of Italy there are conglome- 
rates containing large transported blocks of stone. Fully 
admitting that these were most likely transported by ice, I 
need scarcely remind geologists that no marine remains 
have been found with them, and that they were probably 
deposited in lakes, for although the Miocene boulder beds of 
Piedmont are more than roo feet thick they contain no 
organic remains, and we know that this is a feature of 
modern glacial lakes. The beds rest also on Lower Miocene 
strata, mostly of fresh-water origin. To adduce such 
isolated facts as proofs of the existence of Glacial periods in 
Early Tertiary times is as logical as it would be to argue that 
there is now a Glacial period in the tropics because there are 
glaciers there. It is as if a traveller on the coast of western 
tropical America, coming in sight of one of the snow-capped 
summits of the Andes, should contend—although the sea 
and the land teemed with tropical forms of life—that he was 
in the arctic regions. Probably throughout geological his- 
tory there never was a time when some mountain summits 
did not rise above the limits of perpetual snow, and we may 
expect to find in every geological formation some ice-borne 
boulders, without being forced to conclude that it required 
a Glacial period to transport them. The only safe guides to 
follow are the fauna and flora preserved in the strata, and 
even these fail us when we go far back in geological time, 
for we know not what to call tropical and what temperate 
forms; but so far as Tertiary rocks are concerned we may 
accept their evidence, and they prove that there were no 
oscillations between extreme heat and extreme cold, but a 
gradual and continuous decrease of temperature from the 
Eocene up to the Glacial period. 
Coming to the Glacial period itself, what evidence have 
we of the intercalation of that time of ‘‘ perpetual summer” 
that is, according to Mr. Croll, a necessary consequence of 
his theory? The fact most commonly appealed to, both on 
the Continent and in England, in support of this supposition, 
is the presence of seams of lignite in Switzerland—as at 
Diirnten, in the canton of Zurich—resting on a great thick- 
ness of boulder clay, and capped by beds of gravel with 
large erratic blocks. These seams of lignite generally vary 
from 2 to 5 feet in thickness, but in some parts swell out to 
as much as 12 feet. I admit that the evidence is conclusive 
that after the ice—during the great extension of the Swiss 
