Interglcicial Fossils. — Coleman. 91 
but not by Mr. Townsend, is perhaps preglacial. The fifteen 
or sixteen feet of lacustrine deposit just above the till, corre- 
spond to the Unio- and Pleurocera-bearing beds of the brick- 
yard. 
The section measured by Mr. G. J. Hinde at Scarboro' Hights 
differs more widely from the one described in the present pa- 
per, since there the lowest boulder clay is separated from the 
second by 140 feet of fossiliferous clay and sand.* It is clear 
that our drift deposits vary greatly within comparatively 
short distances. 
In interpreting the facts observed in the Don valley we 
have a first Ice age in which the glacier worked up the Hud- 
son River shales into boulder clay, kneading in a few Arclnean 
boulders, and spreading a tough carpet of till in a gentle slope 
toward the hollow of lake Ontario, and then withdrew; fol- 
lowed by the waters of the lake, which stood 40 or 50 feel 
above its present level, if we suppose no change in the bight 
of land above sea at this point. Mr. Simpson supposes that 
the lake was ice-dammed at this stage, f and drained into the 
Mississippi, making it possible for mollusks belonging to tin- 
Mississippi fauna to invade waters normally flowing into the 
St. Lawrence. The supposition is a natural one, but seems 
contradicted by the fact thatat present the watershed between 
lake Ontario and the tributaries of the Mississippi is several 
hundred feet above the level of the lake. 1 have seen no re- 
ports of buried channels going low enough to drain lake On- 
tario into the Mississippi down to a level only >"><) feel above 
the present, though of course, the possibility of finding one 
cannot be denied. There seems more evidence of an old 
channel leading past Syracuse through the Mohawk valley 
into the Hudson.]; which would not account for the presence 
of Mississippi forms in Ontario, however. In whatever way 
they were introduced, there is no doubt that during intcrgla 
cial time eight or ten species of Unio, several no longer found 
in our waters, lived at the mouth of the Doll, first mi the 
scarcely disturbed surface of the till, afterwards mi beds of 
sand and well rolled gravel brought down by tlie river and 
*Jour. Can. Inst., vol. xv, p. 392. 
tProc. Nat. Museum, No. 952, p. 593. 
JWright, Man and the Glacial Period, p. 202. 
