Ch. XII.] NATURE OF STRATIFICATION. 237 



This fact shows a tendency of the granitic mass to assume 

 that spheroidal form which long exposure to the atmosphere 

 produces on its individual blocks. Now these curved lines or 

 seams, when viewed in a section, occasion some of the ap- 

 parent irregularities observed in this rock: and when they 

 meet with a granitic layer of different composition, they often 

 abut against it, or continue their course in an altered direc- 

 tion, and thus they meet with and sometimes intersect other 

 lines. In this manner, various complicated appearances are 

 produced : but it must be remembered that these are not 

 visible in the perfect rock, as is ascertained, not only at 

 greater depths in the same rock, but when discontinued by 

 coming into contact with a layer not so susceptible of disin- 

 tegration ; yet even in this, similar seams may be detected at 

 or near the surface ; which circumstances appear to show, that 

 these lines depend on the manner in which the materials of 

 the rock have been aggregated together. 



In Cornwall, therefore, the granitic rocks are composed of 

 layers, which, though frequently parallel, yet often exhibit 

 irregularities, like those observed among the adjacent slates; 

 and there is no reason to suppose that this granite is consti- 

 tuted differently from that of other countries. " The size of 

 the concretions of granite and trap," says Macculloch, " if 

 such they are to be considered, is often immense, and some- 

 times resembles strata. It is not often, however, that the 

 laminar form is so perfect, for it will generally be found that 

 the sides of a lamina are far from parallel, and that they fre- 

 quently disappear in their progress, being irregularly en- 

 tangled or implicated with others, not only of different sizes, 

 but of various irregular forms." * In short, are the primary 

 schistose rocks sometimes disposed in regular beds ? So are 

 the different kinds of granite. Are the beds of the former 

 occasionally curved and contorted? So are also those of 

 granite. Do regular schistose beds often abut against rocks 



* System of Geology, vol. i. p. 166 



