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Ch. VI.] STRUCTURE OF THE PRIMARY ROCKS. 91 CHAPTER VI. ON THE STRUCTURE OF THE PRIMARY ROCKS. Two kinds of structure, the internal and concretionary. — Both of these also admit of a binary subdivision. — The concretionary being either simple or compound, — and the internal structure either massive or fissile. — Structure of granitic rocks, — developed by the action of the elements, — can also be ascertained by cleavage. —Macculloch’s remarks on the spheroidal structure of granite — this structure not confined to blocks, but common to the whole mass. — The general structure dependent on the aggregation of individual concretions. — The layers of granite possessed of determinate bearings like strata. — Structure of the primary schists. — The nature and disposition of strata. — The curvatures and convolutions in the beds of the primary rocks.— Their angular and spheroidal structure, resembling breccia and conglomerates. The consideration of the concretionary forms of the primary rocks, and of the manner in which these are aggregated toge ther, so as to constitute considerable masses, may appear to some to be here out of place; but it will be seen hereafter, that this subject will furnish some important arguments, in the discussion of the nature of these rocks : at the risk, there fore, of being deemed unnecessarily prolix, we proceed to offer a statement of facts, in many cases, indeed, of a very elementary nature, but requisite to complete the proposed practical sketch of primary geology. The structure of these rocks may be regarded in two points of view : in the first, as internal; and in the second, as con cretionary. To these might be added a third, the minera- logical; but it is, perhaps, preferable to place this under the denomination of the texture of rocks, comprising the com pound, the homogeneous, the granular, the crystalline, the compact, the fibrous, the scaly, the porphyritic, amygdaloidal, and other textures, on which it will not be necessary to make any farther remarks. The concretionary structure is of two kinds: first, that