THE INSULATION OF ST. MICHAEL'S MOUNT. 13 
The country immediately behind, or north of Marazion, consists 
of Devonian strata traversed by traps and elvans, and attains an 
elevation of about 200 feet. The town stands on a small plain 
terminating in a cliff from twelve to twenty feet high. Judging 
from this cliff, the plain is a sub-aerial accumulation of fragments 
of rock derived from the adjacent hill, and embedded, without 
any approach to regularity of arrangement, in a yellowish clay, 
forming probably no more than from 30 to 40 per cent. of the 
entire mass. 
The most important points in the foregoing description, in 
- connexion with our present enquiry, are, Ist. The materials of the 
Mount. 2nd. The relative level of the isthmus and the sea. 
Ist. The Mount consists almost entirely of granite; a rock 
which all modern geologists hold to be of Plutonic or Hypogene 
origin: in other words, a rock which was not, and could not as 
such, have been formed at the surface of the earth, but was 
elaborated beneath an overlying mass of rock of some other kind, 
which was stripped off by subsequent denudation before any part 
of the granitic mass could have been exposed at the surface of the 
earth. 
The thickness of this superimcumbent mass it is probably 
impossible even to guess, but Mr. Sorby has, by a very refined 
method, estimated the pressure under which the St. Austell granite 
was formed as equivalent to 32,400 feet of rock vertically, that of 
the mean of the Cornish granite to 50,000 feet, and the granite of 
Ding Dong Mine near Penzance to 63,000 feet.* As this pressure 
was in all probability due to the expanding power of heat beneath 
or within the granitized mass, it is not necessary to suppose the 
overlying rocks, the function of which was resistance, had a thick- 
ness even distantly approaching these figures. Nevertheless, it must 
have been very great, and the denudation by which these rocks 
were removed must have been commensurate. 
Should it be objected that, since its solidification, the granite 
has been thrust up through the rocks which formerly overlaid it, 
the veins and dykes which, as already stated, it has sent into the 
surrounding strata, will immediately furnish a conclusive reply in 
the negative. They can be seen extending in unbroken continuity 
* See Quart. Journ. Geol. Soc., London, Vol. xiv, p. 458, &¢., 1859. 
H 2 
