TORONTO GLACIAL AND INTER-GLACIAL DEPOSITS. 637 
neously; one must have preceded the other ; but which came first 
is not easy to decide. 
One might assume that the Scarboro’ clays were formed first, 
a cold climate continuing for a long period of time after the 
departure of the earliest ice sheet ; that the water was drained off 
or the land elevated afterwards, and the Don of those days exca- 
vated its comparatively wide and deep valley apparently not 
greatly different from the present one. Meantime the climate 
had become warm, and southern forms of life pushed their way 
northward and occupied the river and its shores until the second 
advance of ice destroyed them. This hypothesis seems to agree 
with many of the facts, particularly if Dr. Hinde is correct in his 
belief that the stratified clay resting on till near the mouth of the 
Humber is a continuation of the fossiliferous Scarboro’ beds. 
The quite similar clays at Price’s brickyard would then be a rem- 
nant of a wide sheet of such lacustrine deposits afterward eroded 
by the Don. There is an appearance of interbedding of a thin 
layer of the peaty clay at the Taylors’ brickyard with the bowl- 
der-clay above; while the upper fossiliferous beds at Scarboro’ 
were much eroded before this sheet of till was laid down, facts 
which perhaps point in the same direction. If this hypothesis be 
correct the Don beds, being much later than those of Scarboro’, 
may somewhere be found resting unconformably on their eroded 
surface. Up to the present, however, no such section has been 
observed. 
On the other hand one might suppose that the Don beds are 
the older; that after the till was laid down there was a sudden 
change of climate, and that southern forms of life quickly fol- 
lowed up the retreat of the ice-sheet as it vanished under the 
action of warm sun and winds. The fact that Mississippi unios 
lived and died right on the unweathered surface of the blue till 
at the brickyard fits best with this assumption. If there had been 
a long period of erosion before they arrived, one would expect 
to find the till weathered brown and its enclosed pebbles of shale 
crumbled to pieces, instead of being fresh and sharply striated. 
Taking this view, the layer of peaty clay just beneath the middle 
