composition of the former. It must however in candour be added, that after his visit to Auvergne, where he was unexpectedly convinced of the volcanic nature of the products of that country*, Daubuisson hesitated concerning even the basalts of Saxony, and hinted to the author that they might be volcanic, but, as resting on the summits of hills, of an antiquity altogether inconceivable.

NOME V. WACKEN AND CLAY.

This transition has been before described.

NOME VI. JASPER AND KERALITE.

This transition, according to Patrin, is common in Siberia. The author has seen specimens, in the collection of that celebrated traveller, of keralite translucent on the edge, joined with opake jasper. The colours also correspond; but in the keralite they are pale. This transition seems to depend on the greater or smaller quantity of iron, a chief constituent in jasper.

See his papers in the Journal de Physique; and here Dom. I. Mode Basaltin.

NOME VII. SLATE AND CHLORITE SLATE.

This is rather a scarce transition, the latter substance not being common. Slate also passes into mica slate; and sometimes into the massive substance described under the Mode Slate.

NOME VIII. FELSITE AND BASALTIN.

Dolomieu, in his able memoir on petrosilex or felsite, trap, and roche de corne, or magnesian basaltin, observes that they are the chief bases of lavas; and thus entered into his consideration, in forming a system of volcanic productions. He then speaks of the various transitions of his petrosilex or felsite*.

• Journal de Physique, new series, vol. i. p. 250.

"Petrosilex, as I have already said, unites itself by gradual shades with all rocks, in whose composition some of the free earths enter, or compound particles which may assist in the formation of the masses which it chiefly constitutes. Combined with pure quartz, in which it seems to dissolve, it gradually assumes all the characters of quartzose rocks; by a progressive augmentation of talcous earth, it proceeds to unite itself to steatites and serpentines, forming in its progress a kind of fusible jad, which has not the weight of common jad: it acquires the earthy smell, as it approaches the roche de corne; the schistose tissue, in uniting with argillaceous schisti. But it is when it approximates traps, that the shades of its transitions are most insensible: and an infinity of rocks placed between the two, leave the greater uncertainty concerning the species in which they should be classed, as the composition is scarcely ever the same in all the parts of the same mass: one portion shall incline to trap, while the other is affected by the fire like petrosilex. The base of many porphy ries is found in this intermediate situation; as well as most of the ancient grey and green salts which come from Egypt, when it happens that the fineness of their paste no longer allows

ba

the distinct grains of felspar and greenish hornblende to be perceived, which are still visible in the greater number."

NOME IX. GRANITE AND BASALT.

This sometimes occurs in the Egyptian monuments. In Norway, and other primitive countries, veins of basaltin occur in granite; but it is a mere coherence, and there is not the smallest trace of transition.

NOME X. GRANITE WITH GNEISS.

This transition is one of the most common in primitive countries.

Red granite with red gneiss, from the Alps. Grey granite with grey gneiss, from the

same.

NOME XI. GRANITE AND GRANITIC PORPHYRY.

This is also a very common rock.

The passage from granite to granitic porphyry being one of the most remarkable and important, the following observations of Dolomieu will be found to merit particular attention*.

"During the great coagulation, to which the primitive mountains owe their construction, it seems that there have been substances, of which the concurrence, or too great abundance, has impeded or prevented the regular aggregation, in giving the paste a tenacity, in some manner fattening it, to make use of a term applied to mother-waters when they refuse to crystallise. Such are the particles of talc, and of argillaceous and magnesian earths when free. It seems that these earths, naturally unctuous, have prevented the other particles from assuming the places to which the laws of elective aggregation destined them, in causing them to slide on one another. I have pretty generally observed that the superabundance of magnesian earth chiefly acted upon

• Journal de Physique, new series, vol. i. 1794, p. 193.