[HOWARD] SOME OUTLIERS OF THE MONTEREGIAN HILLS 95 



normal essexite of which the nepheline syenite of Mount Royal 

 represents the acid differentiate. Fragments of essexites but none 

 of nepheline syenite have been reported in the breccias, so that they 

 and the basic intrusives with which they are closely related are later 

 than the essexite and probably earlier than or contemporaneous with 

 the nepheline syenites. 



An interesting feature of these occurrences is the presence of 

 monticellite. This mineral was first recognized by Bowen in the 

 alnoite at Isle Cadieux although he also found it in an alnoite from the 

 original locality of Alno. 



This mineral was also found in rocks from Como, Ste. Monique, 

 and possibly in the Husereau occurrence. Bowen states that 

 " it seems not unlikely that the monticellite may be of reason- 

 ably frequent occurrence in alkalic rocks," and points out that pure 

 lime olivine (Ca2Si04) has been found in such rocks. Although the 

 norms as calculated in the Quantitative Classification of igneous rocks 

 are not intended to represent the actual minerals present in the rock, 

 still it seems reasonable to expect that in rocks whose norms contain 

 calcium orthosilicate, there must be a mineral of very nearly the same 

 composition in the rock itself. Otherwise it is extremely difficult, if 

 not impossible, to account for the high lime content and low silica 

 content of such rocks. 



The first intrusion of alkalic rocks in the western part of the 

 Monteregian Hills is represented by the essexite of Mount Royal. 

 This essexite was followed by differentiation products, the more acid 

 differentiate forming an intrusion of nepheline syenite at Mount Royal, 

 while one of the more basic products was intruded as a number of 

 basic alnoitic rocks lying to the west of Montreal with associated dykes 

 and sheets of breccia. These basic rocks are remarkably low in 

 silica and contain the lime orthosilicate monticellite which has hitherto 

 been regarded as of very rare occurrence as a primary constituent of 

 igneous rocks. 



