92 THE ROYAL SOCIETY OF CANADA 



probably the associated alnoites are, therefore, later than the essexite, 

 and may represent a basic differentiation of the main magma of which 

 the nepheline syenite of Mount Royal is the counterpart. 



The only analysis requiring special consideration is that of the 

 biotite-peridotite at La Trappe. It has been shown above that 

 during the formation of the breccias in this locality, the basic constit- 

 uents of the dykes settled toward the main intrusive. Apparently 

 this differentiation went on to some extent in the main intrusive itself 

 and we see by the norm that the rock is very deficient in salic minerals. 

 An increase of less than 10 per cent (an amount approximately equal 

 to the proportion of normative acmite and sodium metasilicate) would 

 bring this rock into the same position as that occupied by the monchi- 

 quite from Ste. Monique. That the analysis is faulty as regards the 

 alumina is doubtful, as the summation is close to 100 and the magnesia 

 is not high. 



On the other hand, this differentiation after intrusion may not 

 have taken place, and the magma may simply have been deficient in 

 alumina. In any case there can be no doubt that this rock is closely 

 associated with the other alnoitic and basic rocks described in this paper. 



Relationships of the Ste. Dorothée Fourchite and Breccias 

 The fourchite sheet at Ste. Dorothée, on the other hand, appears 

 to be very similar in composition to the essexites of the Monteregian 

 Hills. Hornblende, augite, and biotite are always present in these 

 rocks and barkevikite is a common constituent. These minerals 

 form essential constituents at Ste. Dorothée. 



Chemically the fourchite at Ste. Dorothée resembles the essexite 

 of Mount Royal very closely, although it is intermediate between this 

 rock and the essexite of Mount Johnson in the classification. The 

 essexites of Mount Royal and Mount Johnson, the pyroxenite of St. 

 Bruno, and the basaltic vitrophyre from Ste. Dorothée may be classi- 

 fied as follows: 



II. 5. 3. 4. 



Essexite, Mount Johnson 



III. 5. 3. 4. (3. 2. 2. 3) 

 Fourchite, Ste. Dorothée 



III. 6. 3. 4. (2. 2. 3. 2) 

 Essexite, Mount Royal 



IV. 1. 2. 2. 2 



Olivine pyroxenite, St. Bruno 



IV. 3. 3. 2. 3. 



Olivine Essexite, Mount Royal 



