[harvie] palaeozoic BRECCIA OF THE VICINITY OF MONTREAL 255 



The succession, it may be observed, is in harmony with the law of de- 

 creasing basicity. 



With regard to the structure of these hills, it is found that two, 

 Shefford and Brome, are definitely classed as lacoliths (Dresser), while 

 Buchan puts forward the same view for Mount Koyal.^ Mount Johnson 

 and Yamaska on the other hand are typical volcanic necks, with probably 

 Moutarville also. The remaining three have not yet been reported on, 

 although the work on two has been completed. The question of tlie 

 structure is important as showing whether or not to expect the surface 

 phenomena of volcanic activities. For instance, Buchan's suggestion of a 

 lacolith is based partly on the deep seated phases presented by the intru- 

 sives, and on the absence of surface phenomena. On the other hand. 

 Miss Nolan and Miss DLxon from their study of Ste. Helen's Island con- 

 cluded that the occurrence of breccia present there, was originally a vol- 

 canic ash, and that the other occurrences of the district probably had a 

 similar origin. The theory they advance in explanation of the facts, re- 

 quires the assumption that Mount Eoyal was once ean active volcano. It 

 will thus be seen that if the origin of the breccia can be established with 

 certainty, considerable light will be thrown on the past history of Mount 

 Eoyal and its associates. 



Breccia. — A breccia has so far been found chiefly associated with 

 Mount Royal; since, however, this is the most accessible of the Monte- 

 regian Hills, and has thus been the most studied, it is quite possible that 

 this feature may yet be found to be of importance in connection ^vith 

 some of the others also. To enumerate the occurrences of breccia, — there 

 is a small occurrence on the west slope of Montarville, the first hill to 

 the east of Mount Eoyal. Ste. Helen's Island and He Eonde in the St. 

 Lawrence river opposite Montreal are largely underlain by breccia, pre- 

 sumably one mass. Another occurrence was found on St. Paul street, — 

 one of the water front streets of Montreal on the main shore im- 

 mediately opposite these islands, and since large blocks of breccia are 

 commonly dredged from the bottom of the harbour, it is quite possible 

 that this is an extension of the mass forming Ste. Henen's Island and Isle 

 Eonde. These two localities are well out on the eastern slope of Mount 

 Eoyal, but high up on the same side of the mountain a breccia dyke was 

 found when excavating the foundations of the Medical Building at McGill 

 University. Composing the main mass of Mount Eoyal there are several' 

 hills, the southwestern of which is known as Westmount. On the north- 

 east end of this latter there are several small masses and dykes of breccia. 

 Proceeding to the west from Mount Eoyal, breccia is found in numerous 

 localities, namely, at the White Horse Eapids on the Eivière des Prairies, 



2 Canadian Record of Science, 1901, p. 321. 



