The clay-slate of that author, thonschiefer, is our slate, simply so called by way of eminence, but a grand and primitive rock; while the other is understood to be of recent formation.

Uniform.

HYPONOME 1.

HYPONOME II.

With impressions.

The following transitions are upon a larger and more various scale; but may be here subjoined, in order to throw more ample illustration upon a curious and intricate topic.

Saussure has minutely described a singular transition from granite to limeslate, which he observed not far from Courmayeur*.

"Travelling through these pasturages, the eyes always fixed on the primitive chain, I saw below this chain beds similar to slates, and leaning against rocks of granite. As nothing in my mind is more interesting for theory, than the junction of mountains of different orders, I determined to examine this; but as it was too late

§ 872.

in the day, I went to sleep at Courmayeur, distant from it two leagues, and returned on the

morrow.

"Quitting the bottom of the valley, you must ascend for nearly three quarters of an hour, to arrive where the schisti touch the granite. These schisti, which at a distance only appeared a thin surface, adhering against the foot of the mountain, are a considerable mass of different layers. The substance which composes the greater part of these layers is remarkable, in that it briskly effervesces with acids, and yet very easily melts with the blow-pipe into a clear green transparent glass; which runs and sinks on the tube of glass to which it has been fixed.

"Its colour is blackish, and its grain resembles that of a limestone; I wished to see what was the quantity of free absorbent earth that this rock contained: I pulverised 100 grains of it, which I pounded for an hour in distilled vinegar; this acid dissolved the half of it, and those 50 grains were found composed of 44 grains of lime and 6 of magnesia. The other 50 grains which had refused to dissolve in the vinegar, were placed in decoction in aqua regis; being dissolved assisted by heat, 17,47 grains of lime, 2,25 of argil, and 1,42 of iron, were extracted from it, there remained 27 grains and a half of

indissoluble siliceous earth. Uniting the products of these two operations, 100 grains of this schistus were found to contain, Lime 61,45, Silex 27,50, Magnesia 6,00, Argil 2,25, Iron 1,42, Water, air, and loss 1,38. Total 100,00.

"The layers of this schistus are intermingled with layers of a fine sandstone; but little coherent, and which resolves of itself in a white sand, found in quantity at the foot of these same layers. The weak gluten, which unites these grains of sand, is of a calcareous nature.

"These layers are a little bent; but their general position, of those at least which are the lowest, is vertical, excepting by a few degrees, in which they recline against the mountain. There can be no doubt on the position of the beds of these schisti, because they are exactly parallel to the plates of which they are composed. But these layers are cut here and there, and at right angles, by clefts parallel to one another, and which all bend alike, descending to the S. W. under an angle of about 50 degrees. These clefts leave intervals between them; here a foot, there only a few inches. When they are observed at a distance, it is impossible not to take them for divisions of the beds of the rock, so important is it in these researches to see the object close, and observe it in detail; for the in

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terior structure of the rock can alone decide between sections which cross at right angles, which are those which denote the position of the beds. I have already mentioned what I thought of the origin of the fissures which thus cut the beds, and I shall elsewhere refer to it again.

"I have distinguished four very distinct shades in the transition of these schisti to granites.

"The first layers of schistus, where some alteration is observed, assume plates more wavy, brighter, more resembling mica; but they have otherwise the same properties with the others.

"The next are still more waved, plates of real mica are observed, and besides a mixture of quartz, which yields fire with steel, although the rock still effervesces with acids, Veins of a black substance are observed in this same rock, bright, composed of little rhomboids, which appear to be the crystallisation of the purest substance of the schistus; for these crystals dissolve with effervescence in acids, without leaving any perceptible residue; and yet they very easily melt under the blow-pipe into a greenish and transparent glass, which sinks on the point of the glass tube.

"The third shade is a real quartz, mixed with a little mica, and which does not effer

vesce.

"The fourth is a grey granite, with grains of quartz, felspar, and mica.

very small

"This transition in general occupies an inconsiderable thickness; in some places these four layers, taken together, are not more than a foot: nevertheless, the granite does not acquire all its perfection, its grains are not very exact and distinct, till a distance of some feet from its junction. Layers are observed in this perfect granite; they are parallel to all those which form this transition.

Following it round the mountain, I traced this junction of schisti to a considerable distance, by sounding every where with a hammer the bordering beds: I observed no particular difference in the nature of the layers, which form the transition between granite and schistus; but I found some alteration in the position of the beds: advancing towards the S. W. I observed schisti as well as granites overhanging towards the valley, here of S5, there even of 47 degrees. The direction of the layers also changes a little. Those nearest to Col Ferret run to the S. S. W., while those most distant from this same Col, run about 30 degrees more to the west.

"I observed also, in some places, vitriolic effervescences which distilled, sometimes from the schistus, sometimes from the granite itself."