The sand should be coarse-grained and hard.
Of these the most important is the great oval mass of granite in the North, composed of two varieties; one, coarse-grained and older, forms the outside rim, while the fine-grained and newer type occurs in the interior.
Such a rock is typically exemplified by a coarse-grained sandstone or conglomerate, while a limestone may be naturally porous, or, like the Trenton limestone of Ohio and Indiana, rendered so by its conversion into dolomite and the consequent production of cavities due to shrinkage - a change occurring only in the purer limestones.
They vary in texture from a fine-grained compact oolite to a coarse-grained rock composed of angular or rounded fragments, and they commonly exhibit strongly marked false bedding.
On the south coast of the same island are coarse-grained, brownish micaceous and light-coloured calcareous sandstone and marls, containing fossils, which render it probable that they are of the same age as the coal-bearing Jurassic rocks of Brora (Scotland) and the Middle Dogger of Yorkshire.
