Lharv'e] PAL/EOZOIC BRECCIA OF THE VICINITY OF MONTREAL 251 



the Petrographical Museum of McGill University which contains 

 numerous specimens from the various occurrences^ and includes the 

 material used by Miss Nolan and ]\Iiss Dixon in their work. The study 

 has, however, been largely petrographical involving the examination of 

 about eighty thin sections, this work being carried on in the Petrogra- 

 phical Laboratory of McGill University. 



It is proposed in the present paper, — 1. to discuss in detail the vari- 

 ous occurrences of the breccia, correlating them so far as may be possible, 

 both amongst themselves, and in connection with the rocks of the Mon- 

 teregian intrusives; 2. to discuss the origin of the breccia; 3. to enquire 

 into the age of the breccia, more especially in connection with its bear- 

 ing on the history of Mount Eoyal. 



General Geology of the District. 

 Eemarks on the Phisiography. 



The area immediately concerned in the present discussion is repre- 

 sented by a belt about three miles wide running east and west, chiefly 

 across the island of Montreal, with Montar^ille (or St. Bruno) at the 

 east end, and the Oka mountains at the west. The occurrences of breccia 

 all lie within this belt, with the exception of one near Ste. Anne de 

 Bellevue, which is four miles to the south. This district, it will be seeru 

 lies in the St. Lawrence lowland, a plain underlain by Palasozoic sedi- 

 ments, and bounded on the northwest by the older rocks of the Archœan 

 shield, on the southeast by the Appalachian uplift. The sediments dip 

 away from the Archasan, the outcrops of the various formations showing 

 as belts following the margin of the Laurentian. Piercing the sediments 

 at least as high in the geological scale as the Ordovician, are the igneous 

 intrusives of the Monteregian hills, the two most westerly of which, 

 Montarville and Mount Eoyal, lie within the belt just mentioned. These 

 hills are the roots or remnants of an old system of volcanoes, which have 

 more successfully resisted erosion than the sediments which once sur- 

 rounded, and in some instances at least, covered them. An important 

 factor in continuing this erosion was the extensive glaciation to which 

 the district has been subjected, this process tended to remove the 

 sediments somewhat according to the stratification, and in the case of 

 the igneous rocks to produce rounded outlines, rather than the irregular 

 forms typical of decay. The district thus consists of a rather uniform 

 plain broken by several isolated hills composed of igneous rocks, which 

 rise abruptly from it, and which constitute very striking features of the 

 landscape. 



