CONCRETIONARY &C. STRUCTURES OF ROCKS. 167 



of different sizes, but of various irregular forms. It 

 is not unfrequent for these laminae to be curved, so as 

 to have a convexity and a concavity ; 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 ; and, in the same 

 manner, they are sometimes split into imperfect co- 

 lumnar 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 is examined in the chapter on the 

 Destruction of Rocks. (Chap, xiii.) It is proper here 

 to add, that the larger laminar structure is most 

 frequent in granite ; but that it occurs in some of the 

 trap rocks, including porphyries, and is, in particular, 

 very conspicuous in hypersthene rock. The smaller 

 laminae are found principally in the traps and in 

 pitchstones : and it thus appears that this structure is 

 nearly peculiar to the unstratified rocks. It occasion- 

 ally happens that the laminar structure is to be 

 discovered only after exposure to the air, a circum- 

 stance necessarily noticed in the chapter on decompo- 

 sition, and that it may be combined with other varie- 

 ties, as with the columnar, in many of the trap family. 

 It is also found in the veins of the later traps and the 

 antient porphyries, as well as in the products of 

 volcanoes. 



The circumstances thus detailed respecting the 

 rocks to which this structure belongs, added to a 

 careful and unprejudiced eye, must be the Geologist's 

 guide in distinguishing laminae from strata, a concre- 

 tionary form from a real stratification. 



