63 
slopes; on the southern side is a gentle sloping terraced 
platform of drift which in part buries these great blocks, 
forming a “‘tail’’ probably due to the drift accumulating here 
on the lee side of the mountain during the ice movements 
in the Glacial age. This drift, however, has been in part 
at least reassorted by wave-action during the period of 
depression which in this region followed the Glacial age 
and during which the sea covered the plain to a depth 
of several hundred feet at least. On the plain about 
the mountain no rock exposures are seen. A mantle 
of drift covers it, and numerous erratic blocks and boulders 
are scattered about. These are composed chiefly of gneiss 
from the Laurentian highlands, but some of them are 
plutonic rocks from other hills of the Monteregian group. 
On ascending the mountain the first rock which is 
exposed above the drift mantle is a very fine-grained 
dark hornstone, uniform in character and lying in undis- 
turbed 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 
: [ 2} Hid: 
de i ] 
: HT 
i | ie 
Laurvikose {RMR Trensitiona: Rock [fff] Andose =] cosomes Essexose [SS Hornstone 
Diagrammatic section through Mount Johnson, showing the relation of the several 
rock types. 
to the intrusion. This shale, wherever seen, lies flat 
and abuts against the igneous rock of the intrusion, being cut 
off sharply by it, but is not tilted or upturned. The upper 
limit of the shale is shown in the accompanying pro e: 
graph of the mountain. 
The mountain above this hornstone collar is made up 
exclusively 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 colour, of the pulaskite type. This, 
as the mountain is scaled, passes rather abruptly into 
a dark-coloured rock with large porphyritic white feldspars, 
