GEOLOGY AND MINERALOGY. 
75 
altitude of about 2000 feet above tbe sea. Mostly they are filled 
with mud, and altogether present a very ancient appearance. It 
being contrary to all evidence that these rocks were ever in such 
positions as to be bored by pliolades, or any creatures inhabiting 
the sea, we may reasonably form the conclusion that the litho- 
domous perforations which we now find in them have been made by 
the great land snail of St. Helena, the Bulimus auris vulpina, 
previous to its becoming extinct in the Island. This opinion is 
strengthened by the non-existence of any other creature likely to 
have bored them, as well as by the fact that a comparison of some 
of the shells with the holes shows a very accurate correspondence. 
Some additional force may also, I think, be derived from the fact 
mentioned to me by Mr. Gwyn Jeffreys, that this extinct bulimus 
must have lived in water or svvapips, as well as on land. It is true 
that nothing like a lake or a swamp of any size now remains in the 
Island, but there is no reason why, previous to its being so broken 
up and drained by denudation, large ponds and swamps should not 
have existed — indeed it seems most probable that they did, and also 
that their edges or coasts were formed of these very rocks in which 
the perforations occur. The existence of so much moisture would 
have greatly facilitated the passing of the lavas into felspathic 
marls, and thus account for the large masses which occur in a 
tumbled, displaced position from what they must originally have 
occupied as strata. 
The subsequent disappearance of swamps by drainage, as the 
Island became smaller, would very probably account for the death 
and extinction of this bulimus by depriving it of its natural element, 
and perhaps also of its food, in some semi-aquatic plants, the 
existence of which might also be brought to an end as the land 
became drier. 
It is also not a little remarkable that the dead bulimus shells are 
now found on the north-eastern side of the Island, where, no doubt, 
judging from the appearance of the land, swamps would have 
existed at a more recent period than at any other part. 
The Island cannot be said to be rich in minerals, nothing having 
yet been discovered of much commercial value. Pyrolusite, or 
black oxide of manganese, has been exported to Europe and obtained 
a fair market price, but it is an ore of a hard description, and 
difficult to separate from the clay-beds in which it occurs. This, 
