302 
EXPLANATION OF GEOLOGICAL FACTS, 
on my first visit, I felt the fullest conviction that I distinctly traced 
the main body of the schistus sweeping under the granite along the 
margin of the sea ; but on a second visit I could not verify my first 
observation. On my first visit the day was cloudy and the sea quiet ; 
on my second, the sun shone bright, and was reflected by the breakers 
of a troubled sea dashing against the rocks. This important fact, 
therefore, still requires to be substantiated by other observations. 
It is at the junction of the large-grained granite with the schistus 
that any doubts can be felt by the sceptical whether the granite 
overlies the schistus. No doubt can exist of the small-grained 
granite being spread in a thin but extensive sheet over it, as 
the breaking away of portions of this near its centre has exposed 
the schistus beneath. But as small-grained granite has been consi- 
dered of later formation than the large-grained, this fact may be con- 
sidered of little importance. I would venture to observe, however, 
respecting the formation of the small-grained granite at Green 
Point, that it is impossible to avoid the belief that it is coeval with 
the large-grained ; it passes so gradually into it in some places, 
and is so indeterminately mixed with it in others, that the mind 
seeks in vain for any other conclusion respecting it. Granting their 
formation to have been cotemporaneous, it would not perhaps be 
very difficult to explain their difference of character on the Hut- 
tonian theory. It is, I believe, one of the acknowledged laws 
regarding the crystallization of bodies from a state of fusion, that 
their forms are more or less definite in proportion as the rate of 
cooling is slow. If the granite in fusion burst through the schistus, 
the greatest heat must have been at the point of junction near the 
great body of granite ; and here, therefore, the more perfect crys- 
talline forms should occur. The fact is consistent with the theory. 
The component parts of the large-grained granite, when they inter- 
mix with the schistus, retain their crystalline character, giving rise 
to the porphyritic rock before described. 
The sheet of granite, on the contrary, spreading in a thin layer 
over the schistus, would cool with great rapidity and become a gra- 
