30 
ally breaks, but only in high gales from seaward and for a sbort time at 
the highest point of spring tides. The level of the top of the bar exactly 
coincides with the highest dash of the waves. Of course they have piled 
it there, and it stands as an exact measure of the range of their power. 
It is difficult to conceive, if any elevation of the district has taken place, 
why the bar should lie so exactly at this level. ~ In that case it surely 
would have shared in the upheaval and would be found above its present 
position. It would also extend farther inland than it now does. When 
we add to this the fact that wherever the height of the bar can be deter- 
mined, that is, wherever it does not lie against a steep bank, it occupies 
the same position, there cannot remain much, if any, ground for thinking 
that elevation has taken place since the growth of the forest. Taking into 
consideration therefore that subsidence has undoubtedly occurred, and 
finding that there is no evidence of upheaval since the growth of this vegeta- 
tion, have we any clue to guide us in ascertaining the rate of depression ? If 
we assume it to have been uniform, and failing all evidence to the contrary 
we cannot do otherwise, the rate would be found at once, did we know the 
time when the subsidence set in. But this we caDnot at present, at least, 
ascertain. The following suggestions towards it may not however be quite 
useless or uninteresting. 
If St. Michael's Mount be the Ocrinum of Ptolemy and the Ictis of 
Diodorus, which there is little reason to doubt, it was then as it Is now 
an island at high water only. The isthmus therefore must in his day have 
£>een below high water mark. Any depression must accordingly be restricted 
within such limits as will allow this rocky neck of land to have been below 
the upper limit of a twenty-foot tide eighteen hundred years ago, and above 
its lower limit now. That is to say it must not exceed about six feet. 
Taking this as our guide it would give four inches in a century as the rate 
of depression, and if the amount before-mentioned be taken into calculation, 
namely forty-two feet, the time requisite for the accomplishment of the 
change will be twelve thousand six hundred years, a not very extravagant 
period, nor one at all inconsistent with the results of observation made 
elsewhere. 
There is however a considerable amount of evidence in the district, 
tending to show that a much larger amount of subsidence might fairly be 
assumed, but to detail this would exceed the limits of a paper. Such an 
assumption would be in harmouy not only with the results of observation in 
the district but also with the traditions of the county, which unmistake- 
ably point to great destruction of land in pre-historic times. 
