82 THE ROYAL SOCIETY OF CANADA 



As the principal rock type present in this locality is more basic 

 than the other alnoites and associated rocks, and is associated with 

 an acid nepheline rock, it would appear as if some differentiation had 

 taken place before intrusion. This differentiation appears to have 

 been somewhat similar to that at Mount Johnson where the basic 

 differentiate is situated in the centre, and the more acid phases 

 nearer the border. Here, however, the concentric arrangement so 

 conspicuous at Mount Johnson is lacking and the more acid phases 

 appear only as small intrusions around the border of the main mass. 

 The contacts between the different phases is masked by drift. 



Fourchite Sheet and Breccias at Ste. Dorothée 



The southern part of the village of Ste. Dorothée (situated on 

 Isle Jesus about 18 miles from Montreal) is strewn with boulders of 

 a dark grey fine-grained igneous rock. This rock outcrops just east 

 of the last house in the village along the St. Martin road, forming a 

 small knoll 250 yards long from northwest to southeast, and 125 yards 

 in width. It is best developed on the north side of the exposure 

 where it was formerly quarried for road metal. Here the face is 

 vertical, and the sheet is about fifteen feet in thickness. The outlier 

 is in the form of a sill, and the underlying limestones and shales of 

 the Calciferous formation are visible below the igneous rock at this 

 point. 



One hundred yards to the north is a small exposure of the same 

 rocks, while about 200 yards to the north is another similar outcrop. 



In the hand specimen, the rock is dark grey in colour, and small 

 perfectly formed crystals of biotite, hornblende, and augite can readily 

 be distinguished. These are embedded in a light grey feldspathic 

 groundmass. On the south side of the exposure the dark rock is 

 intruded by irregular dykes and stringers of a light brownish grey, 

 feldspathic rock. The contact is usually quite abrupt, and the in- 

 truding rock is very irregular in outline, containing many inclusions 

 of the darker type. Long needles of augite can be distinguished in 

 the lighter intrusive, but the bulk of the rock seems to be feldspathic 

 in character. This phase is well developed along the road, where a 

 cutting has been made through the rock. The lighter rock is of the 

 same nature as the groundmass of the normal rock, and appears to 

 be a later injection which took place just before the original rock had 

 completely cooled. 



As this later injection shows most clearly the character of the 

 groundmass, it is described in detail below. 



