cannot with propriety be arranged under either. The celebrated glazed rock, which Saussure observed near the monastery of St. Bernard, is of this description; and there is a specimen in the author's collection. It has been called an intimate combination of quartz and roche de corne.

rocks.

Most of the Derivative Rocks of Kirwan Derivative belong to this Domain. The name and idea he is said to have borrowed from Bergman. The aggregated stones of Kirwan comprehend granite, gneiss, porphyry, amygdalite, sand-stone, and other substances, visibly compounded of various materials; while his derivative stones he distinguishes from aggregates by this, "that the associated ingredients are not visibly distinct, or at least require microscopes to render them so." He adds, that a derivative stone may be denominated from the species (that is, the Mode), which still predominates; but if it participate equally of both, it may receive its denomination from either. The siderous, siliceous, and argil laceous earths, form the most frequent

combinations; while those of calcareous earth and magnesia are far more rare. In his Geological Essays he observes, that stones are either original, as granite, or derivative, as sand-stone; while, in his mineralogy, he has classed sand-stone, along with granite, among the aggregates.

The appellation and distinction are in fact alike fallacious. That a red sandstone may be derived from the detritus of a red granite, may be justly admitted; but this affords almost the only example of a real derivative stone. And the intimate combinations of which Mr. Kirwan speaks are so far from being derivative, that they often belong to the most original and primitive substances. But when Mr. Kirwan published his valuable system in 1794 (and the last edition is merely reprinted), the knowledge of rocks was extremely confined, and regarded only as an appendage to mineralogy, instead of forming a grand and distinct science, a rank to which its dignity and importance authorise it to aspire.

The term diamictonic, derived from the Greek, implies that two or more substances are so thoroughly mingled, or, in the language of chemistry, so intimately combined, that the rocks cannot be arranged under either Domain, either from preponderance or predominance.

As this Domain depends especially upon the guidance of chemistry, it may be chosen to honour the names of the chief chemists, here arranged in chronological order, from the most ancient to the most modern times*.

NOME I. SIDERITE, WITH SILEX.

[HERMITE, from Hermes, the supposed founder of chemistry, which certainly originated in Egypt.]

Of this kind is the celebrated rock above mentioned, in which atoms of quartz are intimately blended with atoms of siderite; but in

A curious account of the ancient chemists, or alchemists, may be found in the Histoire de la Philosophie Hermetique of Lenglet Dufresnoy, Paris, 1742, 3 vols. 12mo.

Glazed rock.

some portions, as usual in the infinite variety of nature, the quartz will preponderate, and sometimes the siderite. Saussure's description is as follows:

"We now arrived at this singular rock, which formed the object of this excursion. Its superior surface inclincs to the east, under an angle of 43 degrees. It is this surface which is polished, and in so bright a manner, that it forms a perfect mirror. In some parts it is perfectly plane, so that tables might be cut from eight to ten feet in length, and of a proportional breadth; while in other parts it is a little undulated, but still equally polished. It is here veined like a marble; there marked with angular spots, like fragments enchased in a base. The colour varies, the ground being commonly brown or blackish, and the spots of a pure white; sometimes however the ground itself is white. This stone is very hard, yielding abundant sparks under the flint, whence the polish resembles that of an agate or a jasper, having more splendour than that of marble. The white parts are undoubtedly of semi-transparent quartz, infusible by the blow-pipe, but dissolving very speedily, and with a lively effervescence, in mineral alkali. The black parts appear of two kinds; those which are nearest the polished surface losing

their colour under the blow-pipe, and becoming white like the former, but without any further change; and they also melt with effervescence in the mineral alkali, without colouring it in the slightest degree. But in the interior of the stone are found black and soft parts, which, when moistened with the breath, exhale an odour of clay, and melt under the blow-pipe. The black polished parts are therefore also of quartz, or, if you will, of jasper, coloured by some particles of the black pierre de corne, which is found in the interior of the rock."

He supposes that the most natural explanation of the polish is, that it arises from crystallisation on a vast scale, as it is accompanied with streaks, like those common in crystals of quartz.

NOME II. SIDERITE, WITH MICA.

[DEMOCRILITE, from Democritus the philosopher, B. C. 480, who made many experiments on plants and minerals.]

The particles of siderite are sometimes intimately blended with particles of mica.