62 THE ROYAL SOCIETY OF CANADA 



ularly interesting in that the rock contains small veinlets of calcite 

 and biotite. These tiny veins are usually less than two millimetres 

 in width and several centimetres in length and traverse the rock in 

 all directions. In places, small patches of calcite a couple of milli- 

 metres in width and two centimetres in length, occur in the rock, not 

 as veins, but as a primary constituent of the rock itself. The large 

 olivine and augite phenocrysts which are characteristic of the alnoitic 

 rocks of Como and Isle Cadieux are not present in this locality. 



Thirteen hundred yards to the south is a similarly situated 

 exposure which crosses the St. Jerome Road. This occurrence has 

 a crescent-like outline, with the convex side towards the north, and 

 is 600 yards in length by 75 yards in width. Here the coarser 

 principal variety grades into the finer grained minette-like type, 

 the latter occurring as sheet-like bodies two or three feet in thickness 

 in the coarser principal type. There is no clear-cut contact between 

 the two phases and they are simply textural variants of the same rock. 



The next exposure lies 250 yards to the south and is a small knoll 

 around which the St. Jerome road curves on the east side. Two 

 hundred yards to the east of this last exposure is another small hill 

 whose northwest slope is composed of similar rocks. In these two 

 outcrops the fine and coarser types both occur. 



Around the village of Ste. Monique and to the south, the whole 

 area is strewn with boulders of monchiquite, and in the outcrops, 

 several of which occur, both types are found although the coarser 

 principal type predominates. In the southernmost exposure 800 

 yards to the south of the village the fine grained type is lacking. 



This description of the outcrops from north to south is not in- 

 tended to show any particular gradation as the source of the sheet is 

 unknown. The exposures are simply described in the order in which 

 they were visited. 



As all these exposures cover a comparatively small area it is 

 believed that they represent portions of a sheet which originally 

 covered the area within whose limits the present outcrops appear. 

 The roughly horizontal banding of the two types where these are 

 associated together is also significant. The greater part of the sheet 

 was removed by erosion and the remnants acted as a resistant barrier 

 to the eroding action of the ice-sheet and protected, to a certain 

 extent, the softer sedimentary rocks lying immediately to the south. 



The mineralogical composition of the various phases of the 

 monchiquite at Ste. Monique is as follows: 



