OVERLYING AND TRAP ROCKS, 139 



versely, or parallel to the axis of the prism. If often 

 obscure, and limited to small portions of an amorphous 

 mass, it sometimes occupies a considerable space, and 

 is so distinct, that the dark-blue claystones, thus con- 

 stituted, might almost be mistaken for clay-slate ; 

 though, as I formerly remarked, it is seldom percep- 

 tible but on the surfaces under the influence of the 

 weather. It offers another point of resemblance be- 

 tween these rocks and lava, which is also occasionally 

 foliated. This structure is found in basalt, in green- 

 stone, in claystone, in syenite, in porphyry, and in 

 hypersthene rock ; and is therefore not peculiar to 

 any individual, much less to porphyry ; for which 

 reason I have suppressed the term Porphyry slate, 

 contrived by those who did not know a stratified from 

 an unstratified rock, and tending to inflict their own 

 ignorance on others. Those indications of an internal 

 structure, discovered only on decomposition, where 

 the weathered surfaces present a contorted, a spicular, 

 a cavernous, a venous, a scoriform, or a botryoidal 

 aspect, have been sufficiently noticed in the thirteenth 

 Chapter. 



With respect to decomposition, the veins are often 

 more yielding than the surrounding strata, and thus 

 become a frequent cause of caverns and of extended 

 fissures : while, in other cases,, being more permanent, 

 they remain like walls, standing above the surface. 

 Of this, and of the deep-seated decomposition both of 

 the masses and veins, I have already had occasion to 

 speak at some length. From this decomposition there 

 result many varieties of soil, generally distinguished, 

 like the volcanic ones, by their fertility. The best r 

 however, appear to be the produce of the more recent 

 and dark varieties ; and of those, more particularly, 

 which contain conspicuous quantities of calcareous 

 earth. As some of them also have been found to 



