HVPOTHESIS OF CAUSE OF GLACIAL PERIODS 681 
been so fruitfully cultivated by Hinde, Coleman, and others.' 
(1) The time occupied in the ice retreat is there almost without 
second. (2) The duration of the mild climate is recorded: in 
thirty-five feet of clays and sands. It is also implied in the time 
necessary for the migration of the Paw Paw, Osage Orange and 
other trees from more southerly regions to this rather northern 
locality, and also for the migration of the clams and other mol- 
luscs from the Mississippi waters to this rather distant region. 
Both of these migrations were probably rather slow processes. 
(3) The initiation of the returning cold is recorded in 150 feet 
of fine stratified peaty clays and sands. (4) Following this 
there was an unknown period occupied in the transition from the 
conditions of deposition, during which the preceding series had 
been formed, to the conditions of effective erosion which fol- 
lowed. To suppose that this transition was due to the removal 
of an ice-dam that had lingered in the lower St. Lawrence seems 
quite untenable for a long, mild period and a long, cool, but not 
glacial, period had intervened. It was probably due to the cut- 
ting down of the drainage outlet, or to a surface movement, or 
the two combined, and hence probably occupied an appreciable 
time. (5) There then followed a period of erosion comparable 
to that since the last ice invasion. Succeeding this came the 
re-invasion of the ice-sheet.2 These data seem to fairly imply 
that the interglacial epoch represented at Toronto was several 
times as long as the postglacial epoch. 
While nowhere else has so complete a record been found, 
many estimates of the differences of erosion of the several till 
sheets in the Mississippi valley, where the formations are well 
deployed and happily suited to such studies, have been made 
™GEORGE JENNINGS HINDE: Glacial and Inter-Glacial Stages of Scarborough 
Heights. Can. Jour. 1878, p. 388 ef seg. 
A. P. COLEMAN: -Am. Geol., Vol. XIIL, February 1894, pp. 85-95. Ditto. Jour. 
GEOL., Vol. III., No. 6, 1895, pp. 622-645. 
?Canadian Pleistocene Flora and Fauna: Report of the Committee consisting 
of Sir J. W. Dawson (chairman), Professor D. P. Penhallow, Dr. H. M. Ami, Mr. G. 
W. Lamplugh and Professor A. P. Coleman (secretary), appointed to further investi- 
gate the flora and fauna of the Pleistocene beds in Canada. 
