I was struck with the difference between this rock and a breccia at Epinal, in France, where the matrix was a red sandstone, like that of the cathedral at Basle. Here, though the rock had every appearance of having been long exposed to the air, it was as hard as iron; and it was utterly impossible to detach any of the pebbles from the bed: it was difficult even to break the rock at all. I cannot positively state that the gluten in these sandstones is calcareous, but I suppose it to have been so. Compact calcareous rock, as far as I remember, appears to be subject to no injury from the weather. Many churches in Italy, and almost the whole cities of Venice and Genoa, are built of very fine marble; and the perfection of the delicate carvings, however aged, is most remarkable. I remember a church, near Pavia, coated with the finest and most expensive marbles; a range of beautifully sculptured medallions running round its base, though old, were as distinct and fine in their execution as if they had just come out of the sculptor's studio. If, therefore, the gluten of the sandstone be either calcareous or siliceous, it will naturally produce the effect above alluded to, though it is certainly singular that the stone should be soft when first quarried. Sandstone is a rock in which you seldom see many cracks or fissures in the strata: they are generally continuous and solid. Now, there may be a certain degree of density in the mass, which could not be increased without producing, as in granite, fissures running through it: the particles may be supposed to be held in a certain degree of tension, and there may be a tendency to what the French call _assaissement_ (I do not know the English term), which is, nevertheless, resisted by the stone _en masse_; and a quantity of water may likewise be held, not in a state of chemical combination, but in one of close mixture with the rock. On being broken or quarried, the _assaissement_ may take place, the particles of stone may draw closer together, the attraction become stronger; and, on the exposure to the air, the water, however intimately combined, will, in a process of years, be driven off, occasioning the consolidation of the calcareous, and the near approach of the siliceous, particles, and a consequent gradual induration of the whole body of the stone. I offer this supposition with all diffidence; there may be many other causes, which cannot be developed until proper experiments have been made. It would be interesting to ascertain the relative hardness of different specimens of sandstone, taken from different depths in a bed, the surface of which was exposed to the air, as of specimens exposed to the air for different lengths of time. J. R. HERNE HILL, _July 25, 1836._ FOOTNOTES: