114 . THE STRUCTURE OF THE [Ch. VI. 



of this work does not require it ; we shall, therefore, conclude 

 with observing, that the curves are sometimes " simple, like 

 the superficies of a cylinder ; at other times double, like that 

 of a sphere;" and sometimes they are angular or contorted, 

 exhibiting the most complicated sinuosities. And these cir- 

 cumstances of curvature are not confined to a mass of strata, 

 but are possessed by individual strata alternating with others, 

 that are straight and regular ; and may be often observed 

 even in the laminae, or smaller concretions, of which a stratum 

 is composed. 



There yet remains another variety of structure to be de- 

 scribed, not uncommon among the primary rocks, and which 

 is generally considered as a proof of a mechanical origin. 

 Without, however, expressing any opinion on its nature, let 

 us in this place confine our attention to a description of this 

 structure, which is characterised by an aggregation of angular 

 and spheroidal concretions. 



It has been already stated, that, on the large scale, the 

 granitic rocks appear to be composed of spheroidal masses ; 

 and that, during disintegration, the open seams divide the 

 mass into tabular and angular portions: but the structure 

 now under consideration is distinct from this, being confined 

 to the individual concretions of which the mass is com- 

 posed. 



First, as regards the spheroidal or conglomerated struc- 

 ture. An instance of this nature has not as yet been noticed 

 in the Cornish granite, with the exception of those crystalline 

 nuclei, of a dark colour and fine grain, which occur here and 

 there insulated in the granite, and which, indeed, if suffi- 

 ciently abundant to predominate over the containing rock, 

 would exhibit this kind of appearance. In the globular 

 granite of Corsica, however, we have a noted example, in 

 which the constituent minerals are arranged around certain 

 centres in concentric laminae. In the schistose rocks, this 

 structure is of a more frequent occurrence. In the gneiss of 

 Coll, nodules of hornblende-rock are completely enveloped in 



