CHAP. XL 



On the concretionary and crystalline Structures of 



Rocks. 





 THE internal structure forms an important part of 



the natural history of rocks, and is also interesting, 

 from the hints which it may afford respecting their 

 formation, and from the errors to which it may give 

 rise. The modifications of the concretionary structure 

 may be divided into the large and the small, but the 

 limit is undefinable. 



Of the laminar, foliated, and schistose Structures. 



The most important, perhaps, if not the most 

 conspicuous division of the large structure, is that to 

 which the very wide term of laminar may be applied. 

 This is that modification which has so often been 

 confounded, under some of its forms, with the strati- 

 fied disposition ; giving rise, in the cases of Trap and 

 Granite, to serious errors. One of the most interesting 

 varieties in this division, is that which occurs in granite. 

 The size of the concretions, if such they are to be 

 considered, is often immense ; while, for a certain 

 extent, they sometimes put on the appearance of 

 strata so accurately, that it is not very surprising if 

 they have misled incautious observers. It is not often, 

 however, that the laminar form is so perfect ; for, on 

 a careful examination, it will generally be found that 

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

 they speedily disappear in their progress, being irregu- 

 larly entangled and implicated with others, not only 



