STATHAM — ON THE GEOLOGY OF THE SCILLY ISLES. 25 
as well as Mr. Carne, has shown, that though these cracks or joints 
seem at first sight to run in a variety of directions, they are found, 
upon more careful examination, to be reducible, in most granitic 
districts, to three distinct classes ; viz. 1. horizontal, or parallel with the 
grain of the rock : 2. vertical or perpendicular, having generally a 
direction north-north-west and south-south-east ; and, 3. vertical, 
but having a direction from east and west to east-north-east and 
west -south- west. In Mr. Heuwood's report, a tabulated view is 
given of the directions of the joints in the different mines which 
came under his inspection ; but, if I mistake not, no comparison 
is attempted to be drawn with the respective lines of the elvan- 
courses, nor of any indications of the lines of igneous action, 
which may have been presented in the mines. Now, I cannot but 
think it probable that, as in the case before us, if the course of 
the erupted igneous matter were previously ascertained, some 
decided connexion would be discovered between those lines and the 
direction of the joints in the surrounding granite. For what is more 
consonant to reason, than that the heated matter whi^h has at one 
time pushed its way upwards, either filling some existing cavity in the 
granite, or thrusting it aside in its upward course, should so fuse and 
melt that rock, or so thoroughly charge it with heat, as in cooling it 
would possess a tendency to crack in some definite direction and 
according to some definite law 1 It is obvious that, if this were at any 
time the case, the joints or cracks, in whatever direction they might 
be, whether parallel to the line of the heated matter, or at right 
angles to it, would all have a tendency to run the same way ; and, 
thus, wlien decomposition subsequently commenced, and portions of 
the rock began to break away in fragments along the line of joints, 
an appearance of stratification would, in the course of time, be 
brought about, just as we might produce the same appearance in an 
ordinary brick-kiln, by removing tiers of bricks in regular order, 
and leaving others standing in an inclined but orderly succession. I 
do not know whether I shall by this simple illustration render my 
meaning clear, but I cannot help thinking that, in the case of the 
porphyritic ridge of Watermill Bay, the " decidedly stratified " 
appearance of the granite is entirely due to some such action. The 
granite has, in all probability, at some distant time covered the por- 
