the edge of these glaciers appears far
more the result of decomposition than attrition. All finely foliated
rocks, slates, etc., are liable to injury from frost or wet weather. The
road of the Simplon, on the Italian side, is in some parts dangerous in,
or after, wet weather, on account of the rocks of slate continually
falling from the overhanging mountains above; this, however, is mere
disintegration, not decomposition. Not so with the breccias of Central
Switzerland. The rock of Righi is composed of pebbles of different
kinds, joined by a red argillaceous gluten. When this rock has not been
exposed to the air, it is very hard: you may almost as easily break the
pebbles as detach them from their matrix; but, when exposed for a few
years to wind and weather, the matrix becomes soft, and the pebbles may
be easily detached. 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