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A. p. COLEMAN 



referred to as "slate conglomerate," but some varieties of it were 

 described as like quartzite or diorite or greenstone. In fact, the rock 

 not seldom looks so massive in the field that it might easily be taken 

 for a fine-grained basic eruptive, if it were not for the red granite 

 bowlders scattered here and there through it; and some geologists 

 have thought it a tuff or volcanic breccia, just as the Dwyka in South 

 Africa was at first explained. 



In reality, of course, till consists of the more or less finely ground 

 materials of the rocks over which the ice sheet passed. If it traversed 



Fig. 3. — Thin sections of matrix of bowlder clays. Dwyka to the left, Lower 

 Huronian to the right. 



granite, the clayey substratum would be mixed with grains of feldspar 

 and quartz; if it passed over fine-grained greenstones, the particles 

 might suggest an ash rock; if the two materials were mixed, as must 

 have been the case when the Lower Huronian glaciers moved over a 

 surface of Keewatin penetrated by Laurentian granite and gneiss, the 

 resulting bowlder clay would have the composition which we actually 

 find. 



The sections from near Cobalt show a small amount of very fine- 

 grained turbid material, in which a few scales of chlorite can be recog- 

 nized, crowded with angular bits of quartz, orthoclase and plagio- 



