THE PLEISTOCENE OR GLACIAL PERIOD. 491 



beds are underlain by a sheet of bowlder clay which has not yet been 

 positively correlated with its contemporary sheet in the series pre- 

 viously described. It can only be said that it is the equivalent of 

 one of the older drift sheets. The Iowan has been suggested, but it 

 may perhaps equally as well be correlated with an earlier stage. This 

 basal sheet of till is succeeded by a horizon of erosion; and this, in 

 turn, by interglacial beds of stratified sand and clay reaching a maximum 

 thickness of more than 150 feet, the lower portion of which constitutes 

 the Don formation, and the upper portion, the Scarboro formation. 

 Above the latter is another horizon of erosion, which, in turn, is sur- 

 mounted by sheets of bowlder clay and assorted drift, together attain- 

 ing a maximum thickness of 200 feet, and referred to the Wisconsin 

 stages. 



Recalling the ideal succession of faunas and floras of a typical 

 interglacial epoch, viz.: (1) arctic, (2) cold-temperate, (3) warm- 

 temperate, (4) cold-temperate, and (5) arctic, it is to be observed 

 that in the Toronto series the arctic and cold-temperate faunas, which 

 should theoretically have followed the retreat of the earlier ice, and 

 should have been recorded in order above the basal bowlder clay, 

 have not been identified. Their places are perhaps represented by 

 the erosion horizon between the basal bowlder clay, and the stratified 

 sands and clays of the Don formation. 



The latter formation contains a warm-climate fauna and flora, and 

 is, therefore, assignable theoretically to the mild middle part of the 

 interglacial epoch. Up to 1900, the flora of this stage had yielded 

 to the industry of Coleman and others 38 species of plants distributed 

 through 26 genera, as identified by Penhallow. Many of these species 

 indicate a climate appreciably warmer than that of Toronto at present. 

 Among these are the pawpaw, (Asiminia triloba) and the osage orange 

 (Madura arantiaca), which now flourish only in more southerly lati- 

 tudes. The maple, elm, ash, oak, hickory, basswood, etc., were pres- 

 ent, suggesting that this region was then forested with trees of types 

 which now flourish typically farther south. The whole group, accord- 

 ing to Penhallow, implies about such a climate as now prevails in the 

 middle United States, in latitudes 3° to 5° farther south. 



The fauna of this stage contains about 40 species of mollusks, several 

 undetermined species of beetles and cyprids, an undetermined fish, 

 and possibly a mammoth or mastodon, and a bison. Among the 



