387 
Letter Mr Bowdich to Professor Jameson. 
tufa in which the sand and hills are deposited, and in the soil of 
which this small forest must have been growing), which peaks and 
elevations present no traces of sand on their surface, or elsewhere 
above the higher level of that in the flat, i. e. about 250 feet, or 
thereabouts, PL X. Fig. 2. Seeking for that explanation which rests 
on the fewest and simplest causes, it occurred to me, when I first 
reached this bed of sand (which was on the southern side, where 
it is level with the water’s edge), that there might have been no 
irruption or deviation of the level of the sea, but a subsidence 
of the tufa strata, (like that of the shores of Alexandria, which, 
according to Bolomieu, are a foot lower than they were in the 
time of the Ptolemies), the natural consequence of gravity, or 
from one of those slips so frequently evident along its coast, 
which led to a deposit of calcareous sand on the borders of the 
sea ; which sand, from its extremely fine grain, was readily dis- 
persed by the winds, until it reached the north side of the island, 
(for it is barely fths of a mile broad in this part), where the 
drift-line of the sand, with the tufa on which it rests, is about 50 
feet above the sea. But, then, should we find the marine shells 
in such heaps at the height of 250 feet ? Would the sand have 
been so firmly agglutinated as it is in the indurated sheaths 
which envelope the trunks and branches of the trees ? and could 
there be one regular or dip-line descending S. SO E, ? I can- 
not help thinking now, that there must have been an irruption 
of the sea from the northward, covering both this small flat, and 
that already described in Porto Santo, (where a marine shell, 
an Ampullina, is also intermixed with the Helices), and depo- 
siting the bed of sand on both. However, I have performed 
the most important part of my duty, by particularising the fact 
as well as I am able, and will therefore say no more. The high 
tufa cliffs on the north side of this part of the island, behind 
F animal, are broken off abruptly in their whole depth towards 
the sea, and presented numerous dip-lines of strata, deeply in- 
clined to the south from these broken faces, Plate X. Fig. 1. 
as if a considerable part of the island had been broken off, or 
worn away on that side, which would seem to have been formed 
by a crater now lost in the ocean, to the northward. 
Edward Bowdich. 
