A very excellent account is given of the Veryan limestone by S. J.
Trist, esq. in the first volume of the Transactions of the Geological
Society of Cornwall.
Mr. Trist says,
The limestone occurs on the coast at Pendower Beach, and may
thence be traced for a mile and a half inland.
It comes to the surface, in three different places, nearly
equidistant from each other, but at different elevations,
the most inland being probably a hundred and twenty, or
thirty feet above the level of the sea. In each instance it
creeps out at the brow of a hill, and no where appears in
the vale below, where it would seem originally to have
stretched across the valleys, but to have been subsequently
carried away, together with the accompanying matter, by
diluvian action.
In breadth it extends over a superficies of 350 yards, but
alternates with an argillaceous schist, the lime itself
never exceeding three feet in thickness, and that only in
the upper beds of the strata. The lime scarcely amounts
altogether to one eighth of the whole mass.
According to an analysis made by the Rev. William Gregor, a
good specimen of this stone consists of about nine parts in
ten of carbonate of lime.
Mr. Trist then gives a comparative statement of the results from
calcining this limestone, and the well-known limestone of Plymouth,
that 200 Winchester bushels of lime from the kiln, provincially called
shells or foreright lime, are produced from 11 tons of the Veryan
limestone, by the consumption of 46 Winchester bushels of culm, more
universally known as Welsh stone coal; but that 14½ tons are required
of the Plymouth limestone to give the same quantity of lime from the
kiln, with the consumption of 56 bushels of culm, which would make the
Plymouth limestone inferior to that of Veryan, in about 32 per cent.
as to quality, and about 22 per cent. more in regard to fuel. As a
cement, its quality is remarkably good. Small spherical masses of
oxide of iron occur in great abundance; they are, in the opinion of
Mr. Gregor, pyrites in a state of decomposition, the sulphur having
escaped.
The colour of the rock is blue, and it is frequently traversed by
veins of calcareous spar.
In the schist which immediately reposes on the limestone, mica appears
in considerable abundance, and the whole is strongly impregnated with
lime. It is of a soft crumbling nature, decomposing on exposure to the
atmosphere, and in that state it is much esteemed as a manure.
The floor on which the lime rests (probably the whole alternating
formation) is an argillaceous schist, with veins of manganese, which
have been partially wrought.
* * * * *
Mr. Greenough has laid down on his map a broad line extending about E.
N. E. from Gerrans and Veryan, crossing St. Blasey Bay and ending near
Looe, with the inscription, “calcareous matter along this line”.