White-sand Bay, the point where my excursion to this part of the coast ended, and precisely in the meridian of the Eddystone lighthouse, all belong to the grauwacke formation. In White-sand Bay this rock, forming the cliffs, separates by very regular rhomboidal joints. It is of a whitish colour, of a friable texture, like that of the grès-molasse, and might pass for a corneous trap in a state of decomposition. It is accompanied by a rock, having a reddish argillaceous base, containing much oxyd of iron, and fragments of compact limestone, and which effervesces with acids.
There is at Peter Point in St. John's Creek, very near Torr Point, a bed of greenstone, in the composition of which there is a good deal of steatite, completely included in the grauwacke. Though of small extent, it is quarried for building. Not far from this place, on the banks of Lyhner Creek, and on the estate of Sir Henry Carew, there is another bed of greenstone: it is immediately adjoining the ferry. This bed extends from one bank to the other; and on the right or south side of the creek there is a large quarry of it. Very remarkable differences may be observed in the texture of the greenstone, though the specimens be taken from the same bed. The base is sometimes so close, so homogeneous, that single unconnected specimens of it might pass for corneous trap,[1] with pyrites dispersed through it: other specimens, however, taken quite close to the preceding, are of a composition and grain, between that of corneous trap and greenstone: these are in my opinion sufficient reasons for comprehending all the varieties of this substance, under the common denomination of greenstone.
- ↑ It is, more properly speaking, what the German mineralogists call graustein (greystone) which they describe as having nearly a homogeneous base, of an ash-grey colour, and of a dry aspect. Brochant, Traité de Minéralogie, tom. ii. p. 608.
