"The Sandstone which occurs on the northern coast of Cornwall undoubtedly affords one of the most splendid and instructive instances of a Recent Formation upon record. We actually detect Nature at work in changing calcareous sand into stone; and she does not refuse admittance into her manufactory, nor does she conceal with her accustomed reserve the details of the operations in which she is engaged. It does not however appear that any geologist has fully availed himself of so rare an indulgence;—to drop the allegory, no complete or satisfactory explanation has been hitherto afforded of this most interesting formation, nor of the phenomena which attend it. At the period that Dr. Borlase wrote his History of Cornwall, the science of Chemistry had scarcely dawned; we cannot therefore feel surprised at his having attributed 'the concretion of shelly sand to the agglutinating quality of sea water'."
"The sand first appears in a slight, but encreasing state of aggregation on several parts of the shore in the Bay of Saint Ives; but on approaching the Gwythian river it becomes more extensively indurated. On the shore opposite to Godrevy Island, an immense mass occurs of more than a hundred feet in depth, containing entire shells and fragments of clay-slate; and it is singular that the whole mass should assume a very striking appearance of stratification. In some places, it appears that attempts have been made to separate it, probably for the purpose of building, for several old houses in Gwythian are entirely built with it. The rocks in the vicinity of this recent formation in the Bay of Saint Ives are Greenstone and Clay-slate, which appear to alternate. But it is around the promontory of New Kaye, in Fistrel Bay, in the parish of Saint Columb Minor, that the geologist will be most struck with this formation, for here there is scarcely any other rock in sight. The cliffs, which are high and extend for several miles, are wholly composed of it, and are occasionally intersected by veins and dykes of Breccia. In the cavities hang calcareous stalactites of rude appearance. The beach is covered with disjointed fragments, which have been detached from the cliff above, many of which weigh at least from two to three tons. The sandstone is also to be here seen in different stages of induration; from a state in which it is too friable to be detached from the rock upon which it reposes without crumbling, to a hardness so considerable as to require a very violent blow from a hammer to break it;[80] indeed holes are actually bored in some parts for the purpose of admitting cables with which vessels are moored. Buildings are here commonly constructed of it, and the church of Crantock is entirely built with it. By the inhabitants the stone is employed for various articles of domestic and rural œconomy."
"The Geologist, who has previously examined the celebrated specimen from Guadaloupe, enclosing a human skeleton, and which is now in the British Museum, will be forcibly struck with the great similitude which this stone bears to it; and suspecting that masses might be found containing human bones imbedded, if a diligent search were made in the vicinity of those cemetries which have been overwhelmed, I made an excursion with my friend Sir Christopher Hawkins, for that purpose; but time and patience failed us, and the discovery is reserved for some more persevering and fortunate member of the society."
"Such then is the nature and situation of this most interesting formation. In the next place, we have to enquire into the causes which have operated in thus consolidating the sand, and into the peculiar circumstances under which the operation has been conducted."
"It will appear that there are at least three distinct modes by which the lapidification of calcareous sand may be effected, and that the present formation is capable of affording characteristic examples of each."
"The three species of cementing matter to which I allude, are all deposited from water in which they either exist chemically dissolved, or mechanically suspended. The water deriving them from the substances through which it percolates; thus is the first species of cement obtained—
1. By the percolation of water, through a stratum of calcareous sand, by which it becomes impregnated with carbonate of lime.
2. By the percolation of water through strata containing decomposing Sulphurets; by which it becomes impregnated with Sulphuric salts.
3. By the percolation of water through decomposing Clay-slate, or any other ferruginous strata; by which it becomes impregnated with Iron, Alumina, and other mineral matter.
In the first case, the very small proportion of carbonate of lime which is held in solution will necessarily render it a powerful cement, since the density and compactness of a precipitate will generally vary, inversely as the rapidity with which it is deposited. This fact is familiarly illustrated by the obstinate adhesion of calcareous incrustations to the interior surfaces of water decanters. In the second case, wherein a sulphuric salt would appear to act the part of a cement, it may be observed, that the sulphatization of pyrites in the presence of calcareous matter is a very general source of gypsum. The granular gypsum from the Falls of Niagara, which is described by Dr. Kidd as being "as white as snow," owes its origin to a natural process of this decomposition; for I am informed by Dr. Maclure of Philadelphia, who has visited the spot, that it is formed in consequence of the action of water upon decomposing slate, which contains numerous veins of carbonate of lime and sulphate of iron. I have also in my possession a series of incrustations which were taken out of steam boilers in Cornwall, one of which presents an admirable instance of the formation of sulphate of lime, its surface being beautifully studded with its crystals; the water which supplied the boiler, and by the evaporation of which this substance was deposited, was derived from a mine in clay-slate intersected with veins of Pyrites and carbonate of lime."
"With regard to the third species of cementing matter, viz. Oxide of Iron, it is scarcely necessary to state, that in the induration of mineral bodies Iron has been long known to act a very important part; the most superficial observer must have noticed the concretions which so frequently appear on the beach around a rusty nail, or any fragment of iron, while the mineralogist must be acquainted with the proofs which Mr. Kirwan has collected in support of the fact. Nor is the part which it performs in the disintegration of mineral bodies less obvious; by its agency we have seen a loose sand become a hard rock, but if we extend our inquiry we shall find that Iron by attracting a farther proportion of oxygen from air or moisture, soon crumbles into dust, and thus proves the immediate cause of the decomposition of that very rock, of which it formerly constituted the indurating ingredient. In this, as in every other operation, Nature preserves her uniformity, producing the most diversified and opposite effects by the modified application of the same principles."