84 
GEOLOGY OF THE SECOND DISTRICT. 
ported ; and hence, too, when exposed to lateral pressure, they would fall towards each other, 
or pinch in. I do not propose in this place, however, to enter upon the discussion of the 
origin of mineral veins ; and these remarks are intended only to show the general analogy 
between dykes and veins, and that even the diversities which they exhibit may be explained 
without the adoption of two theories for their formation and origin. 
For the origin of trap dykes, it is sufficient for our purpose to suppose that, by some cause, 
cracks and fissures were formed in the rocks subsequent to consolidation, and that they have 
been filled with a thick paste of moulten matter from below. The facts and phenomena which 
may be learnt from a study of this kind of rock, Avill be given more particularly when upon 
the subject of mineral veins ; inasmuch as they are all intimately connected, and require to 
be taken up or considered together. 
2. Porphyry. 
There is but one mass of porphyry in the northern counties, which has fallen under my 
observation. This one is interesting in three respects : 1st, it is connected with the upper 
slates of the Champlain group ; 2d, it presents a false stratification; and 3d, it has produced 
no change in the slates Avhich rest in contact. These points will come in for consideration in 
the subsequent paragraphs. 
This rock is composed of a ground of compact feldspar of a reddish color, in which small 
light-colored crystals of the same substance are sparsely diffused. It is columnar as a whole, 
being divided in the mass into short imperfect columns arranged perpendicularly. The 
most interesting locality of this rock is at Cannon’s Point, about one and a half miles below 
the village of Essex in Essex county. It appears as an intertruded rock between the layers 
of slate, which at some points are disturbed; but in others, there appears very little change 
in the original disposition of the layers, or in their texture. This structure may be observed 
at several other places along the shore in the direction of Split Rock. 
Another interesting feature which this mass presents, and Avhich I have denominated above, 
false stratification, may be seen at the rise in the road towards the village of Essex. It is 
here separated into plates or lamina from one-fourth to one inch in diameter ; which, being 
arranged one above the other like the regular strata or layers observable in all regularly stra¬ 
tified rocks, produce this peculiar character ; and so perfect is the resemblance, that a hasty 
inspection, while passing over the locality, would result in the impression that it was truly a 
stratified rock. In order to correct this view, it would be necessary to observe the disposition 
and characters of the lamina, when it would be seen that the structure is after all columnar. 
There is no interposition of fine and coarse particles, or those peculiar appearances which 
necessarily exist where rocks are formed by the successive deposition of particles, or the 
formation of thin or thick layers according to the amount of matter in suspension at different 
times, or of fine and coarse materials according to circumstances, which must always appear 
in the mass at different periods of its deposition. Besides this, its distinct columnar structure 
at several places where it is exposed perpendicularly on the lake shore by the breaking up of 
