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 above 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 porphyry, 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 structiu'e. The former appears to me the most probable." 
Lign. 3. — Porphyiitic Dyke or Elvan-eouvse, at Wateimill Bay, St. Mary's, Scilly ; visible only 
at low-waler. 
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 ; <? 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 structure of the gi-anite 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 considei's 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. Henwood, C.E., has brought a vast amount of 
industry and experience to the task of unravelling the mystery, in his 
admirable reports upon the " Metalliferous Deposits of Cornwall and 
Devon," read before the Royal Geological Society of Cornwall, between 
1830 and 1836, andjsubsequently publi,shed in a separate volume. He, 
