182 ON THE CONCRETIONARY AND 



or, in other words, that, in the ordinary acceptation 

 of the term, no such thing as a vein exists. The 

 analogy is clearly one of those superficial resemblances 

 calculated to operate only on minds of a similar struc- 

 ture; while, if there is any one fact in geology that 

 is beyond the regions of dispute, it is that of the 

 posteriority of veins to the substances which they 

 traverse. 



A cavernous structure, sometimes rendered visible 

 in sandstones by decomposition, may almost be con- 

 sidered as a variety of this; since the separation of 

 the cells may be considered as formed by such durable 

 intersecting larniirde. The appearances which attend 

 some of these cavernous and reticulating structures, 

 are often very singular; but as they are discovered 

 only by decomposition, they are more particularly 

 noticed in the chapter on that subject. (Chap, xiii.) 

 That they depend on some internal arrangement pro- 

 duced subsequently to the deposition of the strata, 

 can admit of no doubt; but, respecting the nature of 

 this, we must as yet confess our ignorance. 



The fibrous structure is the last which can strictly 

 be enumerated among the concretionary modifications; 

 and it seems to unite them with those which are pro- 

 perly of a crystalline nature. It is known to occur in 

 the carbonates of lime, as in the satin spar and in the 

 limestones of Egg. In the former, it is more deci- 

 dedly crystalline than in the latter, resembling the 

 corresponding arrangement so frequent in gypsum. 

 It is also not very uncommon in the argillaceous 

 schists; in which, as these are not susceptible of the 

 crystalline arrangement, it must necessarily be referred 

 to the concretionary structure. As to other fibrous 

 arrangements seen in rocks, including that which has 

 been called bladed, they arc purely crystalline; their 



