GEOLOGY A ND M IN E R A LO G Y .
New Ground, Horse Pasture, and other portions of the Island, sloped
gently down to the sea without intervening precipices, there doubtless
existed long, low-lying coasts, upon which accumulated a white sand
of broken shells, finer in grain but not unlike that now found at
Ascension. This sand, having drifted into secluded places, and
becoming compressed and hardened, now appears in the form of limestone
beds, fragments of which occur at Lot’s Wife Ponds, Bankses
"Ridge, and Rupert’s Beach. These beds vary very greatly now in
altitude, some being on a level with the sea, others as much as
four or five hundred feet above it. They contain embedded
fragments of very old looking lava rock, and are inclined inland, as
at Lot’s Wife, at an angle of 20°, but whether this inclination is due
to drift or slight upheaval it is difficult to determine. The stratification
of the beds, however, rather indicates the latter. The hardest
of this limestone, when burnt, yields an excellent building mortar,
but it exists in small quantities.
In many respects there exists evidence of the great mass of the
Island having cooled slowly. I t is indicated by the entire absence of
obsidian, as well as by the vitreous edges of the dikes. The hot matter
which formed the dike, coming into contact with the cold sides of
the opening which it filled, would quickly cool and account for the
glass-like edges, in some places passing into serpentine, which occur
in many of the dikes at St. Helena.
Earthquakes happen so rarely, and when they do are so slight,
that they scarcely need be noticed as occurring at all. Four only
have been recorded during the last 370 years—viz., one on the 15th
June, 1756 j another in 1780; one on the 21st September, 1817;
and the last on the 15th July, 1864.
I t is not a little remarkable that in so complete a volcanic formation
no traces of sulphur occur, either native or in combination with
other substances; and of the numerous springs of water which exist
only some few appear impure, and that caused only by the presence
of sea salt, which is plentiful in some of the higher beds of laterite
and rubble. Not a single instance of a spring above the ordinary
temperature is found, and, indeed, no recent signs whatever exist
anywhere to indicate proximity to a volcanic locality.*
* The St. Helena Monthly Register for 1810 states that " a warm mineral spring was a
short time ago discovered near longwood. It rises from under an immense body of soUd rock
its temperature is 66 Falir. which was 6° above the temperature of the surrounding atmo