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Ch. VI.] PRIMARY ROCKS. 93 the granite of Goatfell presents solid and continuous faces of rock; while in others, it offers an irregularly laminated struc ture, very much resembling that which occurs in certain trap rocks, including the syenites and porphyries. The fine grained granite, on the western side of this mountain, in the Glen of Catcol, is very often even schistose. The laminae, into which it exfoliates on decomposition, vary in thickness, but may be found so thin, as not to exceed the tenth of an inch. This granite is principally composed of an intimate and minute mixture of felspar and quartz, with small crystals of hornblende, and occasional scales of mica, sparingly dis persed throughout. The schistose structure, therefore, de pends in no wise on the mica; and presents no analogy to that of gneiss. And it must not be omitted to notice, that this laborious and observant geologist has remarked, “ that the blocks which seem likely to undergo this change, and even those in which it has actually commenced, show no symptoms of future and similar exfoliations; and that all mechanical attempts to produce a new, or prolong an old, fissure, are unavailing; the rock breaking before the chisel or wedge in the ordinary and irregular'manner.” This circumstance is quoted, because it is at variance with the author’s experience: it will, however, be referred to shortly, when the cleavage of granite comes under consideration. The developement of the structure of rocks, by a partial and incipient decomposition, appears to be dependent on the same principle as that by which the crystalline texture of a saline or metallic mass is disclosed by the action of a weak chemical solvent: but the laws, by which this action is go verned, have not been sufficiently investigated. We find, wherever a large section of granite is exposed to the atmo sphere, that its surface becomes divided by two sets of fissures, which preserve an exact parallelism among themselves, and which, crossing each other at right angles, separate the mass into blocks of a quadrangular form. Whilst this process is proceeding, the large square surfaces of the granite will be