218 Canadian Record of Science. 



exclusively of igneous material, which presents a most 

 striking and beautiful instance of differentiation. 



Immediately above the hornstone collar, and in con- 

 tact with it, is a coarse-grained and highly feldspathic 

 syenite, light buff in color, of the pulaskite type. This, 

 as the mountain is scaleJ, passes rather abruptly into a 

 dark-colored rock with large porphyritic white feldspars, 

 which in its turn losses its porphyritic character and 

 passes into a coarse-grained essexite which constitutes the 

 mass of the hill, and which becomes at the summit finer 

 in grain, richer in pyroxene and often holding a little 

 olivine. No sharp lines can be drawn between these 

 several rocks ; one passes gradually into the other, the 

 whole constituting one intrusive unit. The approximate 

 limits of these several rock species are shown in the 

 accompanying map (Fig. 2) and photograph (Fig. 3) of the 

 mountain, it being impossible sharply to delimit the 

 several species, seeing that they pass into one another. 

 The mass therefore becomes progressively more basic as 

 we pass from the margin of the intrusion to its center. 

 The two chief rock types are the pulaskite and the 

 essexite which will be separately / considered. The 

 essexite, being the more abundant rock and one present- 

 ing a greater complexity in mineralogical composition, 

 may be first described. 



Essexite. — The rock is dark in colour and rather 

 course in grain, and although holocrystalline usually 

 presents a more or less marked fluidal arrangement of 

 the constituents. This is especially marked in the zone 

 of transition between the essexite and pulaskite, owing to 

 the presence there of the large feldspar phenocysts 

 which, being arranged with their longer axes parallel to 

 the direction of flow, serve to accentuate this structure. 

 The finer-grained variety forming the summit of the 

 mountain is more massive in character and does not 

 exhibit the fluidal arrangement of constituents. Under 

 the microscope the rock is seen to be composed of the 



