Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/437

 the appearances, which are the subject of the present notice, may be most conveniently observed.

The pebbles of granite, of syenite, and of greenstone, are in a state of greater or less decomposition, but present nothing very remarkable, those of schist are usually soft and rotten, have evidently swelled since the period of their having been deposited here, in consequence of which their laminæ have parted from each other, and the interval is not infrequently filled by calcareous spar, which is occasionally prolonged in the form of thin veins to the distance of a few inches into the sand. The outer surface of the schist is but little changed, but on breaking it, the interior is often found to be little else than a black powder.

The pebbles of quartz, which are numerous, appear to have undergone no change whatever.

The pebbles of limestone are the most abundant of all, and have undergone considerable change. The madrepores and other coralloidal bodies of which the limestone is principally composed, resist the solvent action of the water which percolates through the sand much better than the compact calcareous matter that is interposed between them does; the same is the case with the slender veins of calcareous spar by which the limestone is traversed; hence in those pebbles in which the process of disintegration is only moderately advanced, the surface presents a corroded spungy appearance, the prominent parts being composed of decaying madrepores and veins of calcareous spar, while the interior of the mass is still compact limestone. In other instances, where the decay has proceeded still further, the whole of the interstitial matter is gone, and the madrepores themselves are reduced to a very tender friable mass.

But the substances which have undergone the greatest change are chalcedony and hornstone or chert.