ROCKS OF BLACK AND NIPIGON BAYS. 335 
a base of greenstone.” Still other porphyritic kinds are described as par- 
taking ‘‘ of the character of a syenite.” 
In this a dark-gray mixture of hornblende and feldspar, with magnetic oxide of iron 
and iron pyrites, similar to the greenstone already mentioned, incloses a multitude of 
irregular patches composed of red feldspar and of quartz, generally hyaline, and rarely 
of an opaque white resembling chaleedony. The quartz is also occasionally dissemi- 
nated throughout the matrix without the red feldspar. More rarely red feldspar occurs 
without the quartz, and still more rarely small quantities of caleareous spar are met 
with. The whole mass of the dyke, however, sometimes passes into a uniform small- 
grained mixture of red feldspar and green hornblende with very little quartz, and 
ceases to have either a porphyritice or syenitic aspect. 
Still a third kind of porphyritic dike-rock is described as consisting 
of a very fine-grained mixture of red feldspar and quartz, holding distinct and not very 
large crystals of the same minerals; the quartz crystals being colorless transparent 
hexagonal prisms, terminated by a pyramid at each extremity, and rather uniformly 
disseminated through the mass. * * * 
The greenstone dykes, whether porphyritic or not, possess, without a single ob- 
served exception, a well-marked transverse columnar structure, which is in general so 
truly at right angles to the plane of the dykes that their underlie can be correctly deter- 
mined by it. This structure belongs equally to them, whether their dimensions are 
great or small; but the size of the columns increases with the breadth of the dyke, 
which sometimes attains 200 feet. The number of these dykes is very great: thirteen 
of them, of good size, have been counted in the width of two miles, and their parallel- 
ism for great distances is as remarkable as their number. 
The directions of the greenstone dykes, as well as those of the other descriptions 
which have been mentioned, are in general two, one with the stratification and the 
other transverse, changing with any important change in the general strike; and they 
appear to maintain what might be considered a continuation of these courses into the 
older sedimentary rocks, with a less precise relation to their strike where stratified. 
The point of intersection of the two sets of dykes has been seldom seen. In one in- 
stance, however, on the island of Saint Ignace, a dyke of eighteen inches, coincident 
with the stratification, cuts another of nearly the same breadth running transversely. 
Both of these possess a columnar structure, which has not been observed in the dykes 
of syenitic trap.” 
The dykes in general appear to be more durable than the rocks cut by them, from 
which results a peculiarity in the geographical features of the country. The destruc- 
tive action of the water upon the coast is partially arrested in its progress upon 
meeting with the dykes, and those which run with the strike are in consequence often 
found to shield the shore for considerable distances. They sometimes run out into 
long prongs or promontories, with deep recesses behind them, or present a succession 
of long narrow islands, which act as breakwaters in defending the neighboring main- 
and; and it frequently happens that a narrow breach having been effected in a dike, 
it will be found to be the entrance to a spacious cove worn out on each side in the 
softer rock behind it. 
1 Geology of Canada, p. 72. 2 Geology of Canada, p. 73. 
