[hap.vie] palaeozoic breccia of the vicinity of MONTREAL 271 



A drift covering extends over almost the whole island. In com- 

 position it is an unmodified sandy clay, holding large and small glaciated 

 boulders, principally of Laurentian and Trenton age, though the inter- 

 mediate formations are also represented. Towards the top of the de- 

 posit fragments of breccia are common, as well as a few eratics from 

 Mount Eoyal. 



The Utica formation is only exposed at the south end of the island, 

 the rest of the area underlain by it being covered by the drift. The rock 

 is a dark, almost black, highly bituminous shale, weathering in places to 

 a rusty brown. The ease with which it disintegrates into small thin 

 laminœ, has masked both the strike and dip. However, it is probable 

 that the beds are practically horizontal, or dip to the east at a very low 

 angle. When cut by dykes and sills, a narrow zone of shale in contact 

 with, the igneous rock has been altered to hornstone. With one excep- 

 tion, the contact between the breccia and shale is concealed by drift. 



The breccia, which underlies the remainder of the island is an un- 

 stratified massive rock. It is composed of fragments of rocks, which are 

 angular, subangular, or rounded with facets, but not waterworn. These 

 fragments vary in size from microscopic grains to boulders twelve and 

 fifteen inches in diameter, and the range in age extends from Archaean 

 to Devonian. They are embedded in an extremely fine grained greyish 

 matrix, which weathers to a rusty brown. (See fig. 17.) The rocks 

 represented are red and black shales, hornstone, limestone, — mainly 

 Trenton, red and gray sandstones,— the latter probably Potsdam, quart- 

 5'ite, granite a,?à syenite gneiss. The red shale and red sandstone are 

 considered by Logan to probably come from the Medina.^ 



At one point only is the breccia to be seen in contact with the Utica. 

 The contact] is a brecciated one, the shale being broken up into angular 

 fragments with the intersticial spaces filled with a white yellowish crys- 

 talline dolomitic material, which dissolves out, leaving the shale in relief. 

 Part of this shale has been altered to hornstone. The contact is not 

 sharp, but there is a regular transition from the normal shale through 

 the brecciated facies to the breccia proper. 



In thin sections the brecciation is seen to be very complete, grading 

 down from large fragments, to microscopic dust. There has also been 

 active circulation or mixing of the materials, shown by highly arenaceous 

 fragments being surrounded by those. of the fine grained shale which 

 forms the great bulk of the slide. The fragments are generally outlined 

 by a rim of granular calcite, which being stained with iron stands out 

 very prominently. (See fig. 18.) The filling of the interstices is largely 

 a fine grained almost isotropic igneous material, but in a few instances 



1 Geology of Canada, 18G3, p. 356. 



