24 



THE GEOLOGIST. 



there is what has been called by some an elvan-course, and by others 

 a mass of decidedly stratified granite. This is of considerable length, 

 and rises aboye the granite adjoining it on each side, and seems to lie 

 in thick beds, subdivided into smaller strata, and dipping at a large 

 angle about north-north-west. It is decidedly poiphyry, with small 

 crystals of quartz and felspar. The adjoining granite has likewise the 

 same stratified appearance. The question is, whether the lines of 

 division of the apparent strata are joints, or whether the whole has 

 a slaty structure. The former appears to me the most probable." 



Lign. 3.— Porphyritic Dyke or Elvan-course, at Watermill Bay, St. Mary's, Scilly ; visible only 



at low-water. 



The shaded portion, a, b, c, represents the ridge of porphyry, of lemon-yellow colour at a, or 

 near the sea ; red at b ; and black near the cliffs, at c ; e and/ are ridges of granite, inclined at an 

 angle of about 45 degrees, and apparently stratified. 



Mr. Carne does not enter into any argument to show why he con- 

 siders that the apparently slaty structui^e of the granite contiguous 

 to this porphyritic ridge is due to joints in the rock itself; but I 

 think it will be obvious to any careful geologist that this must be the 

 case, when he considers this simple fact of the false stratification being 

 confined to the granite in the immediate neighbourhood of the por- 

 phyry ; and, secondly, that this appearance is equally visible on both 

 sides of the erupted mass. The whole question of the origin of joints in 

 the granite is one of a most interesting character ; and I believe few 

 localities will be found capable of throwing more light upon the sub- 

 ject than this. Mr. J. Hen wood, C.E., has brought a vast amount of 

 industry and experience to the task of unravelling the mysteiy, in his 

 admirable reports upon the " Metalliferous Deposits of Cornwall and 

 Devon," read before the Royal Geological Society of Cornwall, between 

 1830 and 1836, and subsequently published in a separate volume. He, 



