GEOLOGY AND MINERALOGY. 
47 
to obtain some further information relative to this very remark- 
able submarine ledge, which appears to surround the Island, 
before entertaining the theory of a general subsidence of the 
land, or indeed any theory with reference to the cause of its exis- 
tence.* 
The geological structure of this remarkable land has often been 
curtly described in the few words, “ it is volcanic,” and the explana- 
tion as often considered sufficient, inasmuch as the truth of such an 
assertion cannot be doubted, even by the most casual observer. This 
rocky pile, however, so often briefly dismissed as troublesome to 
inquire into, presents not a few points, in searching out truth, well 
worthy of the student’s attention. The manner of its formation, 
together with the time occupied therein, and the period that has 
since elapsed in bringing it to its present shape and dimensions, are 
each subjects affording unusual interest in reading that page of 
nature’s book which throws light upon the ancient geography of the 
Southern Atlantic region. 
Its isolated position, its peculiar fauna, and its very remark- 
able insular flora, together with its geological character, present 
strong reasons for placing St. Helena amongst the oldest land 
now existing on the face of the globe. It represents a very fair 
type of an oceanic volcano similar to Palma of the Canaries, St. 
Paul’s Island, in the Indian Ocean, and others, of which Sir Charles. 
Lyell makes the following remarks : — “ Every crater must almost 
invariably have one side much lower than all the others, viz., that 
side towards which the prevailing winds never blow, and to which, 
therefore, showers of dust and scoriae are rarely carried during erup- 
tions. There will also be one point on this windward or lowest side 
more depressed than all the rest, by which in the event of a partial 
submergence the sea may enter as often as the tide rises, or as often 
as the wind blows from that quarter. For the same reason that a sea 
continues to keep open a single entrance into the lagoon of an atoll 
or annular coral reef, it will not allow this passage into the crater to 
be stopped up, but will scour it out at low tide, or as often as the wind 
changes.”f There is, in the Island of St. Helena, precisely such a 
* I hope that this subject may be investigated during the present cruise of H.M.S. 
Challenger , as Mr. Gwyn Jeffreys has most kindly promised to bring it before the notice of 
Professor Wyville Thomson. 
t Manual of Geology, by Sir Charles Lyell, M.A., F.R.S., 1855, p. 513. 
