"Rem. This is extremely rare, as the felspar is by far the most subject to decay."

NOME VIII. D. GNEISS.

In this substance, as in granite, the felspar and the mica are chiefly affected. Karsten gives the following examples:

HYPONOME I.

95. Coarse fibrous gneiss, with slightly decomposed felspar, but further decomposed mica; from Swisserland.

"96. Gneiss with entirely decomposed felspar; from the Isaac, near Freyberg.

" 97. Gneiss entirely decomposed, which is scarcely any longer distinguishable, except where the quartz still retains its appropriate structure; with an adhering compound of brown blende, martial pyrites, and some galena; from Freyberg."

The last is properly a vein-stone; and rocks are generally decomposed when in contact with metallic ores.

NOME IX. D. PITCH-STONE.

This substance being of a very compact and unctuous nature, its decomposition seems rather difficult. Among the volcanic specimens from Auvergne, in the author's cabinet, there is a piece of decomposed pitch-stone, which would be mistaken for brown iron ochre, if some parts did not retain their original character.

NOME X. D. SANDSTONE.

These glutenites, whatever be the cement, will decompose into sand. From the appearance of the rocks, in the vast sandy desarts in Africa and Asia, travellers have presumed that those prodigious extents of inert matter proceed from the decomposition of ranges of sandstone. This is perhaps the only decomposition which is destructive of all cultivation. It was natural for an Elector of Brandenburg, the lord of a sandy region, to inquire why God had created sand? While the vast and lofty chains of mountains, covered with perpetual snow, supply perpetual rivers, and perpetual fertility, to the most dis

tant regions; those empires of sand present to human observation no symptom of utility, but, on the contrary, daily encroach on the fertile vales in their vicinity.

Sandstone rock and sand, from the desarts of Africa.

The same, from Arabia. The sand is red and coarse, and the decomposition would appear to proceed from iron; so that a metal of the greatest utility may, in the field of battle, or in the dreary desart, become the most pernicious to the human race.

Sandstone and sand, from the desart of Shamo.

NOME XI. D. CLAY-SLATE.

This is a common occurrence.

Aluminous

slate is particularly subject to decomposition.

NOME XII. D. SAUSSURITE.

This magnesian basaltin, one of the pierres de corne of Saussure, is not only liable to a su perficial decomposition, forming a white crust; but, as it sometimes contains asbestos and ami

anthus, may become rifty, and thus split by the

weather.

Decayed Saussurite, from the Alps.

The same, with amianthus, from the Pyrenees.

NOME XIII. D. MARBLE.

Argillaceous marble, as already mentioned, is peculiarly subject to decomposition. In the north of England, black marble has been observed, accompanied with a soft grey substance called rotten-stone; but this seems rather an adherence than a decomposition. Rotten-stone, though also used in polishing, must not be confounded with tripoli, which seems a mixture of very fine clay and sand, and is only found in veins.

NOME XIV. D. ALABASTER.

In particular circumstances, this substance first becomes of a dull white, and then decomposes into dust.

NOME XV. D. COAL.

This substance, when in contact with what are called whin-dykes, those singular arrects or uprights which sometimes intersect whole mountains, is often observed to be decomposed; having lost its bitumen, and wearing the appearance of being charred. The Neptunists say, that the stone has absorbed the bitumen; while the Plutonists affirm that the melted stone, ejected from beneath, has caused the bitumen to evaporate.

Those immense arrects are often argillaceous, but more generally of a basaltic nature. They are sometimes of prodigious extent; one of them extending from Lothian through the estuary of the Forth into Fifeshire, a space of twelve or fifteen miles. It is observable, that where they intersect the coal, the beds subside in this position: