colouring substance, to behold with astonishment that this base judged to be uniform, is itself a stone composed of two distinct substances, which do not even always require the power of the lens to be observable. Taking, for example, a small piece of the base of antique red porphyry, and with a blow-pipe directing the flame of a taper on it, it becomes brown by the first blast of the fire; and then are easily perceived the small black and white grains, intermingled like those of granite; and continuing the heat. to the fusion of the mass, the white semi-transparent frothy vitrification of the white grains indicates the felspar: the opake black glass produced by the others, announces the schorl; this, more fusible, melts the first, and often encloses small grains of felspar, before the fire has affected them, and then their glasses mingle. As to the proportion of the two substances, it differs; but although I have observed them alternately to take the predominance, the one over the other, in the different masses that I have essaycd, I have nevertheless found that it was the felspar which most often predominated in the base of antique red porphyry."

He proceeds to observe, that what he calls the ancient green serpentine, from the Italian phraseology, and which is our green porphyry,

presents in its base a superabundance of what he calls schorl; that is, the hornblende of the Germans, or siderite of the present work. In some porphyries, called by the Roman artists Ubriagones, the felspar appears, as it were, melted into the base, so as only to present spots of a different tint. It is now well known that the base of the porphyries is a trap, or basaltin; and Dolomieu has the merit of having perhaps first observed that it could not be a jasper, as it is easily fusible by the blow-pipe: but many of his observations will, in the present advanced state of the science, be pronounced to be in

accurate.

Granite and granitic porphyry, from Mount Sinai.

The same, from the Alps.

The same, from the Grampian mountains, in Scotland.

In general the Scotish granites are very irregular; and, in small fragments, often appear as granitels, consisting chiefly of felspar with little seams or particles of mica, while the quartz is often rare and distant.

NOME XII. GNEISS AND MICA SLATE.

This is also a common transition in primitive countries.

Gneiss and mica slate, from the Alps, &c.

NOME XIII. STEATITE AND ASBESTOS.

Steatite, in assuming a fibrous form, passes into asbestos. This transition is very uncommon. Saussure has described a rock of this kind; and Patrin has observed that it affords a remarkable example of the passage of one rock into another.

"This stone, which I received from M. Struve, is of a grey colour, sometimes inclining to yellow, sometimes to green. It greatly resembles asbestos; but the filaments are larger, softer, and more unctuous to the touch; while the fracture lengthwise presents long and large fibres, parallel among themselves, perpendicular to their bases, and irregularly prismatic. Some are straight, others a little bent; and they are sometimes three inches in length. Their lustre is little or none; and where it seems lively, and

almost metallic, this effect is produced by a thin coating of talc, which covers the fibres of the stone.

"The cross fracture is extremely unequal and splintry, with a mixture of spangles of a different substance. This stone is translucent on the edges, to the thickness of four lines, and so soft as to be scratched with the nail, the streak being whitish and of some lustre: it faintly stains cloth with a grey line, is a little flexible, and pretty heavy. Under the blow-pipe it melts into a black globule, not exceeding the tenth part of a line. "It is then evidently an intermediate kind between talc, steatite, and asbestos.

"The long fibres are intermingled with prismatic columns, striated lengthwise, white, laminar, very brilliant, but of which I do not know the nature. They are soft, translucent, and soluble in nitrous acid; but without effervescence, and in length of time. They do not crackle under the blow-pipe; and on charcoal turn brown without melting. They can only be melted on a point of sappare, into brown brilliant glass, without bubbles, and half transparent; the drop not exceeding the tenth part of a line. This stone is found at Weysler Stoude."

* Sauss. 1915.

NOME XIV. SHALE AND COAL.

The particles of shale sometimes pass into coal, or the reverse. But this may rather be regarded as an adherence. Sometimes the shale is marked with vegetable impressions, which likewise pass into the coal.

Coal is sometimes, however, found so impure as to be unfit for domestic purposes; and such mines are commonly abandoned. When in the mineralogic language it passes into slate, it is far from being a recommendation in the kitchen or in the parlour.

The passage of coal into bituminous shale, is the most interesting. The latter sometimes bears the impressions of fish; which never seem Impressions. to be observable on the coal. But Mr. Jameson says that the fish themselves are generally converted into coal, sometimes the scales into copper-ore; bituminous shale being common in copper-mines. It is the slate-clay, Schieferthon of Werner, which generally accompanies coal, and presents vegetable impressions, chiefly of gigantic ferns and reeds now only found between the tropics. This substance is commonly soft; but is sometimes so hard as to resemble basanite.