On the Structures of Rocks 61 



mense ; 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 

 irregularly entangled and implicated with others, not only of differ- 

 ent sizes, but of various irregular forms. It is not unfrequent for 

 these laminae to be curved, so as to have a convexity and a con- 

 cavity; while, in other cases, all their boundaries are convex, 

 causing the laminar to approximate, at length, to a large spheroidal 

 structure. Further, they pass into the cuboidal or square prismatic 

 structure, in consequence of fissures at right angles to their planes. 

 In the same manner, they are sometimes split into imperfect 

 columnar divisions. 



The minuteness of the laminar structure is at times such, that 

 granite possessing this character has been called schistose; but 

 the difficulty which attends some cases of this nature belongs to a 

 division of geology into which I cannot here enter. It is proper, 

 however, to say, that the larger laminar structure is most frequent 

 in granite ; but that it occurs in some of the trap rocks, iucludino- 

 porphyries, and is, in particular, very conspicuous in hypersthene 

 rock. The smaller lamintE are found principally in the traps and 

 in pitchstones : and it thus appears that this structure is nearly 

 peculiar to the unstratified rocks. 



It occasionally happens, that the laminar structure is only to be 

 discovered after exposure to air, and it may be combined with 

 other varieties, as with the columnar, in many of the trap family. 



The circumstances thus detailed respecting the rocks to which 

 this structure belongs, and a careful and unprejudiced eye must 

 be the geologists' guide in distinguishing laminEe from strata; a 

 concretionary form, from a real stratification. 



Ihefoliated structure is distinguished from that properly called 

 laminar, by an undefined, or comparatively unlimited divisibility; 

 and the examples of it are found in the argillaceous schists, in the 

 micaceous schists, in gneiss, and in other analogous primary rocks. 



