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 



