THE LOWER HURONIAN ICE AGE 153 



clase of all shapes and sizes, and with some larger rock fragments 

 such as diabase porphyrite or felsite or porphyritic felsite. A section 

 from near Thessalon, north of Lake Huron, has essentially the same 

 composition, but the minerals have been a little more rearranged. 



For comparison, thin sections of the Dwyka matrix were studied, 

 and were found to be surprisingly like those mentioned above; the 

 only noticeable difference being the absence of chlorite scales, and a 

 somewhat darker and less translucent ground-mass. These slight 

 differences are no doubt due to the fact that the Dwyka conglomerate, 

 being much younger, has undergone less rearrangement. In the 

 illustrations the great resemblance of the two bowlder clays is shown, 

 though by chance a rather fine-grained part of the Cobalt thin section 

 was photographed. 



COMPARISON WITH LATER GLACIAL DEPOSITS 



While much of the Lower Huronian conglomerate has the char- 

 acter of till, there are also phases made up almost wholly of coarse 

 materials, bowlders, pebbles, and smaller bits of rock, with very little 

 of the finer matrix representing clay; and these may be compared 

 with terminal- moraine stuff. 



On the other hand, the tillite sometimes passes into stratified slate 

 with only here and there a bowlder or with no bowlders at all. In one 

 case a bowlder nearly a foot thick was seen with the stratification 

 bending round it as though it had been dropped into mud from 

 floating ice. 



Beside the distinctly till-like and morainic varieties of the gray- 

 wacke conglomerate there are sometimes bands of crowded pebbles 

 somewhat assorted as to size, well rounded, and no doubt water 

 formed, corresponding to the kame materials of Pleistocene deposits. 

 Where stratified conglomerate beds or thinly bedded slates lie between 

 sheets of unstratified tillite, we may safely compare them with the 

 interglacial' beds of stratified sand and gravel between layers of till 

 found at Scarboro Heights, for instance. 



The known thickness of the conglomerate including the other 

 phases mentioned is about 500 feet as measured by Professor Miller at 

 Cobalt, which may be compared with the 400 feet of drift deposits at 

 Scarboro near Toronto. 



