492 GEOLOGY. 



raollusks, 11 species were unios, of which 4 are now living in Lake 

 Ontario, 3 are now living in Lake Erie, but are not recorded from Lake 

 Ontario, and 4 are not known in the St. Lawrence waters, but are now 

 living farther south in the Mississippi basin. 



All these plants and animals had undoubtedly been driven entirely 

 out of the St. Lawrence basin by the previous ice invasion. The inter- 

 glacial interval must therefore have been long enough for a varied 

 fauna, containing many clams and other mollusks, and a complex 

 flora containing many forest trees, to migrate through at least several 

 degrees of latitude. This gives some suggestion of the importance 

 of the interval marked by the erosion horizon below the Don beds. 



Above the warm-climate fauna and flora of the Don beds, there 

 is a cold-climate fauna and flora in the Scarboro beds, embracing 14 

 species of plants and 78 species of animals, 72 of the latter being beetles. 

 This assemblage implies a cold-temperate climate of about the type 

 which now prevails in the region just north of Lake Superior, or that of 

 southern Labrador. The arctic fauna and flora, which should theo- 

 retically have followed this cold-temperate one, heralding the imme- 

 diate approach of the next glacial invasion, is undiscovered. It is 

 probably unrecorded, its time-place falling within the long period of 

 erosion that intervened between the deposit of the Scarboro beds and 

 the formation of the overlying glacial bowlder clay. 



Of the complete ideal series (arctic, cold-temperate, warm-tem- 

 perate, cold-temperate, and arctic), the third and fourth are well 

 recorded, while the rest are probably missing because they fell within 

 the erosion intervals. The later of these intervals, judged by the 

 amount of erosion accomplished, and by the changes of attitude or 

 the cutting down of the basin rim necessary to inaugurate and per- 

 petuate the erosion, are such as to indicate an interval as long as 

 the whole post-glacial epoch. It was therefore quite ample to account 

 for the non-appearance of the later or advancing arctic fauna and 

 flora. The horizon of earlier erosion is less well recorded physically, 

 but if it covers the time of the retreating arctic and cold-temperate 

 faunas and floras, it, too, was doubtless important. 



It is obvious that the record implies a pronounced migratory oscil- 

 lation, but the full measure of this oscillation cannot at present be 

 very closely approximated. The record merely shows that the paw- 

 paw, osage orange, and their mild-temperate associates flourished in 



