96 GEOLOGICAL AREAS OF CANADA. ■ 



the expansion known as Lake St. Peter, the comparatively narrow 

 strip of country between the Laurentian gneissoid rocks and the 

 river margin is occnpied almost entirely by Trenton, TJtica, and 

 Hudson River strata — one or two small exposures 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 con- 

 siderable angles, as near Poiate aux Trembles, Montmorenci Falls, 

 &,c., and their continuity at these spots is more or less disturbed by 

 minor faults. 



As stated above, the 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 fissui-e traversing the district 

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

 tain of Rigaud in Vandreuil, composed partly of a purely feldspathic 

 and partly of a dioritic or hornblendic trachyte, porphyritic in places ; 



(2) the Montreal Mountain, composed essentially of angitic trap or 

 dolerite, but traversed by dykes of compact and granitic trachyte ; 



(3) Montarville or Boucherville Mountain, also essentially trappean 

 in composition ; (4) Beloeil, a dioritic and micaceous trachyte ; (5) 

 Monnoir or Mount Johnson (south of Beloeil), of the same mineral 

 character; (6) Rougemont, in Roiiville 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 metamorphic 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 neighboui-hood 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 rapids 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 



