52 THE ROYAL SOCIETY OF CANADA 



It occurs on the shore of Lake St. Louis, the upper end of the Island 

 of Montreal, the upper end of the islands of Isle Bizard and Isle 

 Jesus, and continues past St. Eustache to St. Lin. This formation 

 also occurs near St. Scholastique, where it divides into two parts, 

 the northern extending to Lachute, and the southern to Rigaud 

 Mountain. 



Chazy Limestone .—The Chazy consists of a series of limestones 

 associated with sandstones and shales. It crosses the Island of 

 Montreal from Pointe Claire to Ste. Genevieve and underlies the 

 central part of Isle Bizard, from which it passes along the north side 

 of the Island of Montreal, spreading out until it occupies the northern 

 half of the island at Montreal. At St. Vincent de Paul it turns north 

 and can be traced to St. Lin. 



Black River and Trenton Limestones. — This formation follows the 

 Chazy from Pointe Claire to Isle Bizard, Isle Jesus, and thence to the 

 southern part of the Island of Montreal, across the lower end of Isle 

 Jesus and to St. Lin. 



Utica Shale. — 'The Utica shales occur on the south side of the St. 

 Lawrence opposite Montreal, as well as on St. Helen's Island, and 

 outcrops at Point St. Charles. A patch of metamorphosed Utica shale 

 occurs on the southern slope of Mount Royal near the Upper Reservoir. 



Lorraine and Richmond. — These formations lie to the east of the 

 Utica, the strata around St. Bruno being late Utica or early Lorraine, 

 probably the latter, while the Lower Richmond is found at St. 

 Hilaire Station. 



The Silurian and Lower Devonian (including the Oriskany) are 

 only represented as inclusions in the breccias associated with the 

 Monteregian Hills. 



Monteregian Intrusives. — Of the main intrusives of the Monte- 

 regian Hills, only the two western ones, namely, Mount Royal and 

 St. Bruno, require mention here as the remainder of the group lie 

 well to the east of the area under consideration in this paper. 



Of these Mount Royal is, in all probability, a volcanic neck or 

 plug; while St. Bruno has been described as a laccolith although the 

 evidence is not conclusive.^ 



At St. Bruno there was a single intrusion of a basic rock (pyroxe- 

 nite) consisting of "biotite, hornblende and augite with basic labra- 

 dorite and olivine occasionally in considerable amounts." Mount 

 Royal is chiefly composed of two distinct rocks representing at least 

 two periods of igneous activity. The earlier (essexite) intrusion is 

 composed of " Labrador ite, reddish-violet augite, brown horn-blende 

 'Geology of St. Bruno, by J. A. Dresser, G.S.C. Memoir No. 7, 1910. 



