!58 The Ajticfican Geologist. April, is&o 
EDITORIAL COMMENT. 
Causes of Glaciation. 
In ascertaining the causes of the climatic changes which 
have permitted the accumulation of glaciers and ice-sheets 
(m large areas of the present torrid and temperate zones, two 
great periods exceptionally characterized by glaciation arc; 
to.be considered, occurring respectively near the close of 
the Paleozoic and Tertiary eras. Although millions of years 
separated these glacial periods, there was no similar exten- 
sion of glaciers and ice-sheets during that very long interval, 
nor probabh' at any time preceding the Permian glaciation. 
The evidences of that early glacial period, found in the trop- 
ical regions west, north, and east of the Indian ocean, were 
well stated by David White, with the bibliography of this 
subject, in the American Geologist for May, 1889 (vol. iii, 
pp. 299-330); and the\' are again brought to our attention in 
the present number by Prof. Hitchcock, and by the review 
of Dr. Molengraaf's recent study in the South African Repub- 
lic. 
The causes of the widely separated Permian and Pleisto- 
cene glaciations appear to be indicated by other geologic 
conditions belonging to the same periods. Their great epei- 
rogenic movements, with the upfolding of long and massive 
mountain ranges, were probably sufficient, by the altitude 
given to the areas which became ice-enveloped, to account 
for their climatic changes and glaciation. This view was 
published by the present writer ten }'ears ago, as an appen- 
dix of Wright's "Ice Age in North America;" and again five 
years ago it was included in the ninth chapter of "The Gla- 
cial Lake Agassiz" (Monograph XXV, U. S. Geol. .Survey). 
It appeals to deformations of the earth, giving high altitudes 
of large areas by lateral pressure of the crust, previous to 
the birth or further growth of mountain ranges; and it is 
consistent with the doctrine of the general permanence of 
the continents and occcins, because the lateral pressure could 
be relieved only by uplifts of parts of the continental areas 
with corresponding depressions of parts of the ocean basins. 
Support of this explanation of the causes of accumulation 
of ice-sheets seems to me to be given by the close associ- 
