42 THE CANADIAN NATURALIST. [Vol. vi. 



white brick, while the marine Leda clay burns into red brick. 

 The chemical cause of this I have already referred to, but 

 whether it implies that the inland clays are fresh-water, or only 

 that they have been derived from a different material, is uncertain. 

 The gray clays of the Hudson River series in Western Canada, 

 might, according to Mr. Bell, have afforded such clays. 



Under the theory of a glacial sea immediately succeeding the 

 elevated Pliocene land, the great amount of decomposed rocks 

 which must have accumulated upon the latter constitutes an im- 

 portant element in the estimation of the rate of deposit of the 

 Erie and Boulder and Leda clays. It is also to be observed that 

 this glacial sea might have had to scour out of the lake basins of 

 Canada only the soft mud of its own deposition, the rock-excava- 

 tion having apparently been in great part effected in the previous 

 Pliocene period. On this subject I find that Dr. Sterry Hunt 

 had, before the publication of Dr. Newberry already alluded to,* 

 shown that not only channels but considerable areas about Lakes 

 Erie and St. Clair had been deeply excavated in the palaeozoic 

 rocks and filled with Post-pliocene deposits. The Devonian 

 strata, he remarks, " are found in the region under consideration 

 at depths not only far beneath the water level of the adjacent 

 Lakes Erie and St. Clair, but actually below the horizon of the 

 bottom of these shallow lakes." He shows that around these in 

 various localities the solid rocks are only met with at depths of 

 from one to two hundred feet below the level of the lakes, while 

 "the greatest depth of Lake St. Clair is scarcely thirty feet and 

 that of the South-western half of Lake Erie does not exceed 

 sixty or seventy feet, so that it would seem that these present 

 lake basins have been excavated from the Post-pliocene clays, 

 which, in this region, fill a great ancient basin previously hollowed 

 out of the palasozoic rocks, and including in its area the South- 

 western part of the peninsula of Ontario." 



It would thus appear that in the Pliocene period the basin 

 of the lakes may have been a great plain with free drainage to 

 the sea. Whether or not it was afterwards occupied by a glacier, 

 this plain and its channels leading to the ocean were filled with 

 clay at the beginning of the Post-pliocene subsidence ; and at a 

 later date the mud was again swept out from those places where 

 the Arctic current could most powerfully act on it. 



(^To he continued.^ 



* On the Geology of South-western Ontario. Am. Jour. Sci. 1868. 



