378 O. E. LE ROY — GEOLOGY OF KIGAUD MOUNTAIN, CANADA 



Rougemont, Beloeil, Montarville, Mount Royal, and Rigaud. The hills 

 of Brome, lying south of Shefford, and mount Johnson, soutli of Beloeil, 

 are on another line, but evidently belong to the same series. Sir William 

 Logan has described them as being of post-Silurian age, and from the 

 field relations of those reeentl}^ examined such has been found to be the 

 case, with the exception of Rigaud, Avhose age is doubtful, owing to 

 the fact that the contact between the Paleozoic and igneous rocks is 

 wholl}'' concealed by drift. In the absence, then, of any direct proof 

 such as a contact would afford, it was thought by the writer that possibly 

 the petrographical character of the rocks would show Rigaud to be closel}^ 

 related to the other hills. With this end in view, a detailed examina- 

 tion of Rigaud mountain was made, and the following pages contain the 

 results of the field and laboratory work, together with the conclusions 

 arrived at. 



Topography 



Rigaud, owing to the general levelness of the immediate surrounding 

 countr}^ occupies a more promnient position than its height would other- 

 wise warrant. In sha[)e the mountain is roughly oblong, and has an 

 area of about 15 square miles. The topographic features closely resemble 

 those of the neighboring I^aurentian — rounded ridges and knobs of rock, 

 parti}' bare, partly wooded, with drift-floored valleys of varying width 

 between. The marginal area and some })arts of the interior are well 

 forested, otherwise the rock is exposed or only covered by a scrubby 

 growth. In the west the mountain consists of a series of interrupted 

 ridges, and longer axes of which have an east-and-west trend. The cen- 

 tral part is rather plateau-like in character, and is made up of subor- 

 dinate elevations of the roche moutonnee t3'})e, which exhibit in many 

 instances stoss and lee slopes. On the line between Saint George and 

 Sainte Marthe the plateau terminates in an abrupt slope to the south, 

 and on this line the highest ])oint of the mountain is situated, there 

 attaining a height of 750 feet above sealevel. From this slope south- 

 Avard the area is but little higher than the immediate surrounding 

 country. The eastern part is marked by one ridge, which is continuous 

 throughout the width of the mountain, and runs in a northeast direction. 

 The valle3^s between the i)lateau and principal ridges are comparative! 3" 

 broad, shallow depressions, well drained by intermittent creeks into the 

 rivers a la Graisse and Racquette. The3'' are floored with gravel, sand, 

 and sandy loam, which in recent cuts show a very even stratification. 



The most prominent ridge is the one fronting on the Ottawa river, the 

 western part of which is shown in figure 1, plate 33. Its average eleva- 

 tion is about 550 feet, but attains at its peak a height of 704 feet above 



