51 



known, occur among the other rocks of that age in the Lake 

 Superior region ; while in New Brunswick Messrs. Bailey and 

 Matthew report 1 the occurrence of well-marked conglomerate 

 or breccia in each of the three groups which they have 

 established among the Huronian rocks of that province ; 

 hence I conclude there is, on- general principles, no a priori 

 improbability in the supposition that the pebbles in our Hu- 

 ronian petrosilex were deposited cotemporaneously with the 

 other materials composing this rock. There are many instances 

 where the second of the hypotheses above stated is an adequate 

 explanation of the phenomena in question. The pebbles in 

 these cases are usually obscure, often escaping observation 

 on the first view of a fresh or polished surface. They pos- 

 sess a special interest at some points, as throwing light on the 

 origin of a peculiar variety of banded petrosilex, and will be 

 noticed again farther on. Limiting our attention now to the 

 more conspicuous of the included pebbles, we find that, as a 

 rule, they are either too rounded, or differ too much in color 

 and texture from the enclosing paste, to be brought within 

 the scope of the regelation theory ; and although the first 

 hypothesis is beset by no special difficulty, save that it in- 

 volves the existence of some older petrosilex from which the 

 pebbles were derived, I am constrained to believe that we must 

 look to the third supposition for the true explanation. For, so 

 far as I am aware, those limited portions of the Huronian 

 petrosilex containing distinctly marked pebbles occur chiefly in 

 the immediate vicinity of the petrosilex breccia of the Shawmut 

 group ; and in nearly every instance the evidence is plain that 

 the petrosilex has experienced some extravasation, and has, 

 consequently, been in a more or less fluent state. The point 

 where the enclosure of pebbles in Huronian petrosilex is least 

 open to doubt is on the eastern shore of Marblehead Neck. 

 The rock exposures here are exceptionably good, and afford 

 ample facilities for the study of the mutual relations of the 

 Shawmut group and the Huronian system. The former re- 



1 Geological Survey of Canada; Report of Progress, 1870-71. 



