OF CENTRAL CANADA PART V. 341 



posures of the Potsdam formation on the St. Maurice and at St. 

 Ambroise alone representing the lower beds as seen west of the 

 Chicot fault. In this eastern portion of the district, the strata are 

 tilted in many places at considerable angles, as near Pointe aux 

 Trembles, Montinorenci Falls, etc., and their continuity at these spots 

 is more or less disturbed by minor faults.* 



As stated above, the Lower Silurian strata of the more southern 

 and western portions of the Upper St. Lawrence district are broken 

 through in places by trachytic and trappean masses, forming a series 

 of isolated mountains which rise above the generally level surface of 

 the country to elevations of from 600 to 800 feet. Most of these 

 occur apparently upon a single line of fissure traversing the district 

 in a general south-easterly direction. They comprise : (1) the 

 Mountain of Rigaud in Yaudreuil, composed partly of a purely 

 feldspathic % and partly of a dioritic or hornblendic trachyte, porphy- 

 ritic in places ; (2) the Montreal Mountain, composed essentially 

 of augitic trap or dolerite, but traversed by dykes of compact and 

 granitic trachyte ; (3) Montarville or Boucherville Mountain, also 

 essentially trappean in composition ; (4) Belceil, a dioritic and 

 micaceous trachyte ; (5) Monnoir or Mount Johnson (south of Beloeil), 

 of the same mineral character ; (6) Rouge mont, in Rouville County, 

 a trappean mass like that of Montreal in general composition ; and 

 (7), the Yamaska Mountain, essentially a micaceous trachyte. The 

 Mountains of Brome and Shefford belong to the same eruptive series, 

 but lie within the crystalline district to the east. In addition to 

 these principal masses, many dykes of similar character traverse the 

 surrounding strata ; and some of these in the neighbourhood of 

 Montreal and Lachine are intercalated with the soft shales of the 

 Utica series, which have become more or less worn away, leaving the 

 associated trap bands in the form of projecting ledges. Most of the 

 rapicfs in this part of the St. Lawrence have been thus produced. 



The superficial deposits of the district comprise Glacial boulders and 

 related clays and gravels, with Post-Glacial and recent accumu- 

 lations. Drift or Glacial deposits, proper, are of general distribution ; 

 and in some places, as on the Rigaud Mountain, the boulders form 

 roughly parallel ridges, several feet in height. The Glacial strise 

 of the country have two prevailing directions south-west and south- 



* The fossils in these various Lower Silurian formations are practically identical with those of 

 the same formations in Ontario : see ante, pages 315 to 317. 



