thin layers of sandstone and sand, followed by a bed of coal, in some places not less than 28 yards in thickness. Immediately upon this coal reposes a platform of basaltin, forming the level on the summit, which is about nine miles in length and about three broad. The basaltin exceeds a hundred yards in thickness.

"The grunstein appears almost every where above the basalt, and in some places has the appearance of a beautiful granite; the grains of siderite being black or green, laminar, and as large as peas, while those of felspar are whitish. On the lower part of the platform, towards the west, there is a basalt in prisms; the most black, the most compact, and the most homogenous, as far as can be judged, that can well be observed. I here arranged the sequence of a dozen speci mens, which presented a decrescent progression, with regard to the size of the grain, from the beautiful grunstein to the compact basalt, of which I bave spoken; and to shun the objection that the specimens did not belong to the same continuous mass, I chose some in which the small grained grunstein was in the midst of the compact basalt; and they might be seen, so to speak, melting into each other." He then quotes the remarkable passage of Dolomieu in Ancient basalt. these terms: "I have seen many statues, mor

tars, sarcophages, made of black stones, which have all the characters attributed to the ancient basalts, and which have preserved that name; and I can say, with positive certainty, that none of them is volcanic." Dolomieu then proceeds to state that some of them are siderite, or massive hornblende; but the most common are a kind of granite, in which the siderite so predominates that the mass appears black, though it be associated with a white felspar, of which the grains are so small, or so interlaced with the scales of siderite, that it is difficult to discern them; especially as the felspar itself sometimes appears black, because it is transparent. He adds that it happens sometimes that a greater quantity of felspar imparts to the rock the appearance of a real granite; that is, as Daubuisson observes, a real grunstein.

This passage of Basaltin into the real Basalt of the ancients, is one of the most remarkable in Geology; and particularly interesting to the accurate and scientific observer. It seems, however, to be somewhat surprising that, while these substances are often found to coalesce, the Egyptians did not prefer the close grained and uniform basaltin to their coarser basalt. Siderite is also found in Mount Sinai, and perhaps in the eastern chain between Egypt and the Red Sea;

but as the ancient authors are unanimous that the basalt came from Abyssinia, it probably occurred under the appearance of columns, of too small a diameter to be employed in architecture or monuments. It is to be regretted that the mountains of Abyssinia have not been explored by any geologist, as the transcendent beauty of the emerald-green granite alone might invite a research into that interesting region.

NOME III. BASALTIN WITH PORPHYRY.

Basaltin being the base of porphyry, it is natural to expect many examples of this kind. Among others, near the village of Renaison, in the department of the Loire, there occur, after passing through fragments of granite, rocks of black trap, surmounted by porphyry of the same base, the transilience being clear and palpable. This porphyry is crowned by another porphyry, of a brownish grey; but in this the crystals of felspar are long, and thinly scattered (a porphyron); while the others are white, and frequent. The black porphyry, and even the grey, are barder than the trap.

The separation of the trap or basaltin from the porphyry is clearly marked by an undulat

ing line, in a fragment which has been polished. The porphyry has taken a fine polish, while the basaltin remains dull. The polish of the porphyry has brought to light little crystals of schorl, or siderite, which could scarcely be discovered in the rude fragments.*

Werner's

account.

NOME IV. BASALTIN AND WACKEN.

This transition has also been accurately traced by Werner himself. Speaking of the mountain of Scheibenberg, he says, "I have seen there, in a successive series of shades, the most perfect transition from clay to wacken, and from this to basalt (basaltin): these three substances are the produce of the same formation; that is to say, they are precipitates or sediments of the same dissolution, which becoming more and more quiet, has deposited the clay, then the wacken, and lastly the basalt." This explanation depends upon Werner's theory, that the rocks were deposited by waters in different states of agitation or of tranquillity. It may be added, that there is much heat, or, in strict terms, caloric, in water itself, which would otherwise be in a state

Journ. des Mines, iv. 133.

+ Daub. Basaltes, 58.

of ice, not to mention the heat developed by crystallisation; so that the agency of heat may be conceived as admitted even by the Neptunists.

volcanic.

On the transition between Basaltin and Wacken, the remarks of Daubuisson may also be adduced. "We have already observed that basalt has great connexions with the argillaceous rock called wacken. Let us recollect those prisms, of which one of the extremities is a true basalt, while the other is an argillaceous substance, both being the evident produce of one effort; a circumstance which excludes every suspicion of a volcanic origin. This argillaceous Basaltin not wacken cannot be considered as arising from an eruption of mud; for between it and the basalt there is a most marked transition, there not existing even a line between them. Nor can it be said that this wacken is a decomposed lava; for at Scheibenberg, for example, the wacken passes to common clay, which degenerates into sand, and then into gravel; but a lava, when decomposed, does not produce gravel of quartz.' He adds in a note, that olivine, augite, &c. though common in the basalt, are not found in the wacken; so that the latter cannot be a de

* Daub. Basaltes, 73.