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 yariety 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. Henwood'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 Unes 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 which 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 *? 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, when 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- 



