26 THE INSULATION OF ST. MICHAEL'S MOUNT. 
and from 60 to 70 in the second, above the bottom of the adja- 
cent valleys in the same vertical plane; and the implement- 
bearing deposits which they contain were carried into them when 
the bottom of the valleys was very little below the entrances. 
In other words, since the existence of Man in Devonshire, the 
Brixham Valley has been deepened, by excavation or re-excava- 
tion, as the case may be, to the amount of at least 100 feet ; and the 
Ilsham Vale, adjacent to Kent’s Hole, to the extent of upwards of 
60 feet. But as the bottom of the former, so far as it is known, is 
not the limestone of the district, nor, in the ordinary meaning of the 
term, rock of any kind, but an undoubted portion of the submerged 
forest of Torbay, it is obvious that the time expended in excavat- 
ing the valley below the level of the cavern, fills but a part of 
the interval which separates the era of the Cave Men of Devon- 
shire from the present day. In order to obtain the whole, we 
must add to this part the time represented by the lodgement of 
the blue forest clay of Devon, or the tin-ground of Cornwall; to this 
again must be added the period in which the forests grew; to 
this a further addition must be made of the time during which 
the entire country was carried down at least 70 feet vertically, by 
a subsidence so slow, and tranquil, and uniform that it no where, 
throughout the area of Western Europe and the British Islands, 
disturbed the horizontality of the old forest soil; and, finally, we 
must also add the time which has elapsed since—a time which, of 
itself, thanks to the description of St. Michael’s Mount by Diodorus 
Siculus, we know certainly exceeded 2,000 years, and which the 
volume of the stratified deposits overlying the forests, as well as 
the amplitude of the existing foreshore, warrants our believing 
exceeded it by a very large amount. 
To me, the Insulation of St. Michael’s Mount in Cornwall has 
its chiefest interest in this connexion. It is the first, or most 
modern, of a series of trustworthy stepping-stones leading back- 
wards towards the far reaching Antiquity of the Human Race. 
