92 THE STRUCTURE OF THE [Ch. VI. 



which is exhibited in the integrant masses, of which these 

 rocks are always composed, and which possess forms more or 

 less determinate, such as cubes, rhombs, and spheroids ; and 

 which may, therefore, be termed the simple concretionary 

 structure : secondly, the compound, or that which results 

 from the aggregation of simple concretions, giving rise to 

 layers, beds, and other forms of rock-masses, which often par- 

 take of the same figure as the parts of which they are com- 

 posed. 



The internal structure is that which is brought to light 

 when we attempt, by violence, to subject the simple concre- 

 tions to a farther subdivision : it is also of two kinds, the 

 massive and fissile ; and generally displays the texture of the 

 rock when this has been obscured by the decomposition, which, 

 almost universally, has taken place on the sides of the con- 

 cretions near the surface of the earth. The massive and 

 fissile structures mutually pass into each other ; and certain 

 stages of this transition have been distinguished by geologists, 

 under the terms tabular, lamellar or laminar, foliated, and 

 schistose. This transition is not a mere abstract idea, for it 

 actually occurs in the same rock, as in the case of greenstone 

 and other hornblende rocks ; and many instances have been 

 already mentioned of the passage of lamellar and foliated 

 into compact gneiss, not to be distinguished mineral ogically 

 from granite. With these preliminary remarks we shall now 

 proceed to an examination of the structure of the granitic 

 and schistose primary rocks. 



The internal structure of the granitic rocks is generally 

 massive, but not always so. The fine-grained granite of 

 Castle-an-dinas, and of other parts of Cornwall, may be easily 

 broken into thick laminae, as may also several kinds of pro- 

 togine, of eurite, and of felsparite ; and more particularly the 

 latter, which sometimes shews a tendency to a schistose struc- 

 ture. A remarkable occurrence of this kind, in the Isle of 

 Arran, is recorded by Dr. Macculloch. * In many places, 



* Western Islands of Scotland, vol. ii. p. 346. 



