«2.. Fat quartz, in which a few particles of red felspar are imbedded.

3. Quartz and felspar united in equal portions, rather in short stripes than in grains, with a few minute spangles of mica. The different components being in very small quantities, constitute a body which, at first sight, appears homogenous.

"4. Quartz and felspar, in such minute grains as to resemble a sand-stone.

"5. Red compact felspar? In this I cannot, even with a good lens, distinguish any admixture of quartz; but when held in a particular direction, the silver mica is visible. I conjecture this to be of the same nature as the preceding, but to be composed of much minuter parts.

"6. Red granite, or rather felspar and quartz, forming a vein or stripe in spatous (granular) hornblende; which is likewise interspersed with red particles of felspar.

"7. Two stripes of the preceding granitoid mixture, separated by brownish mica.

"8. Stripes of the preceding granitoid mixture imbedded in, and separated by, a greenish mass, probably of the nature of hornblende.

"9. Red felspar, in irregular spots or blotches of the size of a large pea, and in smaller particles, in greenish spatous hornblende.

"10. Black spatous hornblende, interspersed with small particles of red felspar.

❝ 11. Fine grained black spatous hornblende, interspersed with very few and very minute particles of reddish felspar.

"12. A brown stone, and, to the naked eye, almost homogenous; but which is a mixture of nearly equal portions of red felspar and black hornblende; but both in very minute particles. "13. Black-grey wacken.

"14. The same, with a spot of siskin green lapis nephriticus, or kind of jad.

"15. A mixture of hornblende and the same lapis nephriticus, with some quartz, all so intimately mixed as to form nearly a homogenous basis or ground, in which are small streaks and particles of red felspar.

"16. Reddish grey petrosilex, including a few particles of pellucid felspar.

"All these specimens are from about three or four miles of the centre of the chain; the other parts of it I never examined.”*

Townson's Tracts, p. 216.

the subject.

THE decomposition of rocks forms a Importance of striking feature in geology, as a great part of the productive soil, and many of the substances used in important manufactories, may be considered as chiefly derived from this circumstance. Several of the most useful clays are reputed by some to be merely decompositions of felspar; the

Loam.

Mould.

mixture of sand being a decomposition of quartz. Bergman found the loam near London, to contain only 13 of argil; the remaining 87 being a redish grey sand, as fine as flour. What is called mould, consists chiefly of vegetable and animal remains. The fall of leaves in a forest creates a fine black mould.

In various parts of England, and other countries, the loam is of a red colour, and proceeds in what may be called belts or zones (for strata can only be superimposed on each other) for a great distance, but with various interruptions. This red tinge can scarcely arise from the decomposed felspar of red primeval granite, as some have supposed; for in that case the hardest nodules of the granite would probably still be found, as in the red sand-stone; but may merely proceed from the admixture of red oxyd of iron, while in other spots the black oxyd may predominate. Argillaceous earth is found in the most primitive substances; and theory can scarcely be expected to determine whether the fertile

clay, which forms so prodigious and important a portion of the surface of this globe, and furnishes aliment to animals and vegetables, arises from a decomposition effected, during myriads of ages, by the superincumbent waters; or by a mere deposition from the original mass and constitution of the waters themselves.

On the decomposition of rocks, the observations of a skilful chemist must be particularly exact and interesting, for which reason those of Mr. Kirwan are extracted; more especially as they abound with examples which are essential to the nature of the present work. It It may also be prefaced, that the decomposed rocks have never hitherto been treated in any profess ed work of mineralogy, so that the novelty of the subject calls for every aid of illustration.

separa

"Decomposition consists in the tion of the constituent parts of a stone, or other substance; and may be either total or partial. Disintegration denotes the se

P 2

Kirwan's explanation.