the laminar texture of felspar, causing its loss, without depriving the felspar of the faculty of assuming the exterior forms of its usual crystallisation. This is perceived in those felspars, which constitute the large spots in green porphyry, called serpentino antico; and still more in the felspars, which mingled with green hornblende form the granites called Egyptian greens. It frequently happens that their compact fracture no longer presents any indication of a laminar texture, though they still affect the quadrangular prismatic form, which belongs to their mode of crystallisation.

"Just as in the magma of mother-waters, reduced to a state of paste by evaporation, there are particles which, escaping from the viscidity of the medium in which they are engaged, aggregate and form crystals, which are found buried in the mass: in the same manner, in these kinds of magma of the great precipitation, it is rare that some isolated crystals are not found among them; and which have acquired so much more bulk and regularity, as they have had more facility of aggregation. They are distinguished from the paste which contains them, by their form, their tissue, and almost always by their colours, brighter than that of the base. Thus are formed rocks called porphyries; and

which, in reality, only differ from granites by this accident of aggregation*.

"The distinction established between granites and porphyries is proper for common use, it is necessary for artists; nevertheless the lithologist could not admit it in a strict sense, without exposing himself to an error, which might lead him to mistake the identity of the origin of these two rocks, and the analogy of their composition. The celebrated naturalist (M. de Saussure), who has furnished us with a great and important truth, by proving, by a thousand excellent observations, that the parts of granite are contemporary, that they have all been formed in the same element, and by the same cause, and that the principle of this formation is crystallisation; but who has thought he ought to make two separate genera of granites and porphyries, and who to distinguish them has said, in granite there is no paste, which envelops the stony grains of which it is composed, while in porphyries, is seen a uniform base, or cement, in which the other stones are enclosed this naturalist, I say, by the progress of his researches, has soon himself found the insufficiency of these distinguishing characters, of

:

This can only apply to granitic porphyries: and some other remarks must be pardoned, from the state of the science at that period.

which I have long combated the precision. Primitive mountains have often shown him, as well as myself, many rocks which have united the two modes of being, and which seemed to be intermediate species between real granites and real porphyries; and to point out the gradations by which nature passes from the formation of the one to the other. How many rocks have I not observed, which, by their polished surfaces, showed the texture attributed to porphyries, by distinct and isolated crystals, forming spots on a base apparently compact, and of a different colour; while their fracture represented grains of granite, by the scaly tissue of the substance which had appeared to be the paste, in which the other substances were enveloped; for granites have a granular appearance, not always by the detachment of the grains of each of the substances which compose them, but by the nature of the texture of the felspar, of which the plates cross each other when confusedly crystallised*: and in all compound rocks, the substance which

"It is equally on account of their scaly tissue that sparry marbles, called saline, seem formed of large grains, adhering together by juxtaposition. They owe the appearance of it to a confused crystallisation, which interlaces the sparry plates; and they lose this granular aspect, to assume that of a compact and uniform mass, when they are deprived of this commencement of regular aggre gation."

is sufficiently abundant not to be divided by the rencounter of other small stones mixed with it, and for its parts to form a kind of continuity of mass, in surrounding the other substances, of which the grains are easily isolated, may be considered as the principal base of the rock, or as the cement which agglutinates the small stony bodies, of a different nature, concurring to the formation of the mass. Such are granites, where felspar alone often constitutes three-fourths, sometimes four-fifths of the mass; and if an abstraction of the sparry tissue is allowed, which depends on a rather more perfect aggregation, and of which it may be deprived without changing its nature, the granular appearance of the granite disappears, the felspar assumes the aspect of a cement in which the other stones are enclosed, and the rock acquires the conformation of porphyry, without the transition of the one to the other requiring any other condition. Nature often, as if she would demonstrate the identity of the two rocks, performs herself, in certain masses, this successive transformation of granite to porphyry, by taking away and returning at intervals its laminar tissue to the felspar; and she produces masses which, according to the expression of definitions, may be in part placed among granites, in part among the genus of por

phyries. It is not even requisite that the felspar should entirely lose its texture; it is sufficient that it be in very small plates confusedly intermingled, and that it contains other crystals of the same nature, but larger and better marked, and a little distinct by their colour from the base in which they are contained. Thus there is often observed among the Egyptian monuments, at Rome, a rock whose base is a mixture of felspar and black hornblende, both in small grains, although still very apparent; in this kind of granitose paste are contained tolerably regular large crystals of white or red felspar, which form spots on the base of the rock, and which give it the greater appearance of a porphyry: as sometimes the abundance of hornblende renders the paste which contains these crystals almost entirely black*. The granites called the green of Egypt, composed of hornblende and felspar, become similar to a porphyry, if the proportion of hornblende ever so little exceeds that of the felspar; because then the crystals of the latter detach themselves from one another, and, by separating, form distinct white spots on the dull green base of the rock. The uncertainty of the

Dolomieu by no means excels in literary composition, his sentences being very tedious and complex. His long notes, which only distract the attention, are here thrown into the text.