THE MONTEREGIAN HILLS 



257 



form in character and lying in undisturbed horizontal beds. It 

 can be seen at intervals all around the base of the mountain, 

 forming a sort of collar, and is undoubtedly a shale such as that 

 usually constituting the Utica formation, here however altered 

 by its proximity to the intrusion. This shale wherever seen lies 

 flat and abuts against the igneous rock of the intrusion, being 

 cut sharply off by it, but not tilted or upturned. The upper 

 limit of the shale is shown in the accompanying photograph of 

 the mountain. 



Fig. 3. — Mount Johnson, as seen from the southwest, showing limits of the several 

 rock types composing the mountain. 



The mountain above this hornstone collar is made up exclu- 

 sively of igneous material, which presents a most striking and 

 beautiful instance of differentiation. 



Immediately above the hornstone collar, and in contact with 

 it, is a coarse-grained and highly feldspathic syenite, light buff 

 in color, of the pulaskite type. This, as the mountain is scaled, 

 passes rather abruptly into a dark-colored rock with large por- 

 phyritic white feldspars, which in its turn loses its porphyritic 

 character and passes into a coarse-grained essexite which consti- 

 tutes 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 



