14 THE INSULATION OF ST. MICHAEL’S MOUNT. 
from the central mass into the beds they have invaded, and thus 
show that the latter have participated in every movement the 
granite may have undergone. 
The geologist, therefore, so far from having the least doubt 
that there was a time when the Mount was not that kind of half- 
island which it is at present, cannot but recognize an era when it 
was completely and deeply encased, and the space between it and 
the lofty hill behind Marazion was filled, with a very different 
kind of rock. 
2nd. We turn next to the relative level of the isthmus and the 
sea. From the description already given, it is obvious that if the 
district were raised 13 feet the isthmus would be constantly above . 
the sea-level, and the Mount a permanent peninsula; whilst, on 
the other hand, if the district were lowered to the extent of but 
7 feet the isthmus would always be under water, and the Mount 
for ever an island. To prove that the district was once at a greater 
height above the sea than it is at present, it is only necessary to 
appeal to the submerged forest long known in Mount’s Bay. As 
has been already pointed out, such a forest is conclusive evidence 
of subsidence ; and it is difficult to suppose that Leland—the first 
author who mentioned it—could have failed so to regard it. 
Indeed, he tells us that “ Ther hath been much land devourid 
betwixt Pensandes and Mousehole. Ther is an old Legend... . 
a Tounlet in this Part (now defaced and) lying under the Water.”* 
In other words, a townlet or village, not destroyed by the mere 
encroachment of the waves, but permanently overwhelmed. We 
have already seen that he elsewhere mentions “as a token of the 
Grounde wasted” the occurrence of the “Rootes of Trees, in dyvers 
Places betwyxt the Mount and Pensants, near lowe Water Marke ;” 
hence, even if the trees originally grew at the sea level, and even 
if there are none further sea-ward, the district must have been 
during the growth of the forest 18 feet higher than it is now—an 
elevation which must have kept the isthmus permanently dry, 
even if there had been no soil on it, and have deprived the Mount 
of its present semi-insular character. 
The detailed description of this forest by Dr. Borlase, who 
carefully investigated it in January, 1757, confirms Leland’s state- 
* Itinerary, Vol. iii, p. 17. 
