On the Norian or " Upper Laurentian" Formation. 183 



with that which to the north cut off the limestone and lay 

 unconformable - upon the Grenville series. 



Instead of this, we have in reality the Grenville series 

 throughout the entire area broken through in places by anor- 

 thosite masses which often follow the strike of the gneiss 

 and appear to be interstratified with it. 



Although at many points on the boundary between the 

 anorthosite of the Morin area and the surrounding gneiss, 

 both rocks come in contact without any alteration of the 

 gneiss being visible, yet at a few places, especially between 

 Shawbridge and Chertsey, a dark heavy and somewhat 

 massive rock rich in bisilicates and often containing a 

 little quartz and some unstriated feldspar appears at the 

 contact of the anorthosite, and may possibly be a contact 

 product. The boundary of the typical anorthosite against 

 this rock is generally quite distinct, whereas the latter 

 passes gradually into the gneiss of the district, so that 

 it is difficult to decide whether it represents a distinct 

 and abnormal variety of the gneiss, or a contact pro- 

 duct of the gabbro. The same rock, or at least a very 

 similar one occurs largely developed, at the northwest 

 corner of the area, between the typical anorthosite and 

 the gneiss, and appears here to be a peculiar variety of gab- 

 bro since it is nearly or quite massive and often shows a 

 distinct "schlieren" structure. It cuts through the gneiss 

 but seems to be continuous with the anorthosite. Continuous 

 outcrops of the two rocks which would make it possible to 

 determine their relationship have as yet nowhere been 

 found, but there is evidence to prove that it is a part of the 

 anorthosite mass, and not a separate intrusion, although the 

 transition is a rather sudden one. 



The anorthosite, mass is cut through in many places by 

 coarse pegmatite veins. These are especially abundant 

 about the edge of the area where they break through the 

 gneiss as well as the anorthosite. In mapping the anor- 

 thosite, it was frequently possible to surmise an approach 

 to the limits of the area from the appearance of numerous 

 pegmatite veins. They are, of course, by no means exclu- 



