CRYSTALLINE STRUCTURES OF ROCKS- 181 



into botryoidal forms, has been observed in certain 

 limestones, as in that of Sunderland. A similar ar- 

 rangement is occasionally found in the sandstones; 

 and sometimes, in the red varieties, it is indicated by 

 the presence of white spheroidal spots, or portions. 

 Of these, no explanation has been suggested, and I 

 have none to offer. 



The spheroidal structure of the oolithe limestones 

 of England, appears to be merely an aggregation of 

 rounded grains, and requires no notice. That of the 

 pisolithes, which consists 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 pearlstone, as it will be noticed in its 

 proper place hereafter. (Chap, xl.) 



Of the venous> cavernous, Jibrous and scaly 

 Structures. 



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

 surfaces have been exposed to the weather, they 

 present a reticulated appearance, as if from the in- 

 tersection 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 in- 

 terior; the whole mass presenting an uniform texture 

 and colour. This peculiarity is very frequent in 

 granite; but it occurs also in gneiss, in micaceous 

 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 similar origin-, 



