[HOWARD] SOME OUTLIERS OF THE MONTEREGIAN HILLS 53 



and brown mica, while olivine, titanite, apatite, and other accessories 

 are often present; nepheline is present only in small amounts, while 

 hauyne can occasionally be detected." The second intrusion, consist- 

 ing of nepheline syenite, took place on the northern border of the 

 essexite, and is composed essentially of orthoclase, nephelite, and green 

 hornblende with small quantities of plagioclase, pyroxene, garnet, 

 and nosean with other accessory minerals. 



At St. Bruno, and more particularly at Mount Royal, the in- 

 trusions were followed by the formation of great masses of breccia 

 in sheets and dykes, including fragments of the sediments surround- 

 ing the igneous core ; also there are complementary sets of dykes cutting 

 all the earlier rocks and one another. 



The breccias have been investigated by Dr. Harvie, who de- 

 scribes occurrences from the west side of St. Bruno, St. Helen's Island, 

 and Isle Ronde in the St. Lawrence River opposite Montreal, from 

 St. Paul Street, and the site of the Medical Building of McGill Univer- 

 sity in Montreal, and also on the north-east end of Westmount 

 Mountain. West of Montreal, breccia is found in numerous localities, 

 namely, at the White Horse Rapids in the Riviere des Prairies, at 

 several places on Isle Bizard. and on the eastern slopes of the Oka 

 Mountains at La Trappe. To these may be added an exposure of 

 breccia near St. Anne de Bellevue, described by Logan, and two 

 mounds to the north-west of Ste. Dorothée, on Isle Jesus, which are 

 described below. 



Harvie notes in his paper (p. 277) that: "An important fact 

 revealed by the pétrographie study is that in every case the cement 

 of the breccia was in a molten condition both before and after the 

 inclusion of the fragments. . . . The peculiar position of the blocks 

 which are the sole representatives of the Helderberg and Oriskany 

 in this district, has been the source of frequent speculation. Since 

 it has now been shown that the paste was in a molten condition when 

 it enclosed the fragments, and that the breccia, as a whole, has acted 

 as an intrusive, the explanation is rendered comparatively simple. 

 The breccia represents the truncated pipe or outlet of a reservoir of 

 molten material, which outlet may have reached the surface and even 

 formed a subsidiary cone to Mount Royal, or else it may have been 

 of the nature of a laccolithic mass not opening on the surface. In 

 either case, the intrusion extended up into the Helderberg and Oris- 

 kany, which must have overlain the Utica . . , " He also shows 

 that as the breccias of Isle Bizard and Westmount Mountain contain 

 inclusions of an altered product of the essexite, they are later than 

 the essexites. 



