72 Dr. Mac Cullbch on the Concretionary and 



The spheroidal structure of the oolite limestones of England 

 appears to be merely an aggregation of rounded grains, and requires 

 no notice. That of the pisolites, which consist of crustaceous 

 agglutinated spherules, is probably the result of a deposition from 

 water ; the exact nature of which is not very apparent. I shall 

 here forbear any remarks on the spheroidal structure of pearl- 

 stone, as it was treated of in an article formerly published in this 

 Journal. 



Of the Venous, Cavernous, Fibrous, and Scaly Structures. 



In many rocks it may be observed, that where the surfaces have 

 been exposed to the weather, they present a reticulated appear- 

 ance, as if from the intersection of veins, of a nature harder than 

 the general mass of the rock. On breaking such rocks, however, 

 no corresponding appearances are found in the interior ; the whole 

 mass presenting an uniform texture and colour. This peculiarity 

 is very frequent in granite ; but it occurs also in gneiss, in mica- 

 ceous schist, and in the sandstones. It has been conceived to 

 arise from some original structure, but is at best a very obscure 

 circumstance. It deserves notice, perhaps principally, because it 

 has been used as an argument to prove that all veins are of simi- 

 lar origin, 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 ones calculated to operate only on minds of 

 a similar structure ; 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 considered as a variety of this ; 

 since the separation of the cells may be considered as formed by 

 such durable intersecting laminae. The appearances which attend 

 some of these cavernous and reticulating structures are often very 

 singular ; but as they are only discovered by decomposition, they 

 belong more properly to the history of that process. That they 

 depend on some internal arrangement produced subsequent to the 

 deposition of the strata, can admit of no doubt; but, respecting 

 the nature of this, we can only as yet confess our ignorarce. 



