I fib CHAP. XI. 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