4194 DIALLAGE ROCK. 



also it varies much, even within very narrow limits. 

 The texture is often confusedly crystalline, like that of 

 granite ; the rock breaking indifferently in any direc- 

 tion, though commonly with great difficulty, on ac- 

 count of its extreme toughness. This is more parti- 

 cularly the case in the small-grained varieties, and in 

 those where compact felspar is an ingredient : the 

 larger-grained kinds, and those containing common 

 felspar, being comparatively frangible. But it is often 

 fissile in one direction, while it yields with difficulty 

 in the opposite one ; the texture resembling that of 

 gneiss, or being imperfectly schistose, and the fissility 

 arising, similarly, from a parallel tendency in the 

 crystals of the diallage. It often also contains those 

 veins called contemporaneous, in which the two mine- 

 rals are intermixed in large portions or irregular cry- 

 stals, or in which one of the constituent minerals exists 

 to the exclusion of the other. 



The stratification of the Diallage rock of Unst and 

 Balta is so obscure, that it might, on a superficial 

 view, be considered as an unstratified rock, analogous 

 to granite. But more careful observation discovers that 

 it is disposed in protuberant masses, in lines parallel 

 to the general direction of the neighbouring strata, and 

 that all the points projecting into the sea, preserve the 

 same bearing. This disposition is further evinced in 

 its regular juxtaposition to the strata of gneiss, and of 

 micaceous and argillaceous schists, which it follows, 

 and by its occasional alternation with these rocks, how- 

 ever its scanty exposure in the interior, and the lowness 

 of the shores, may deprive the observer of the unques- 

 tionable evidence derived from the forms and divisions 

 of the beds ; which are also obscured by the innume- 

 rable fissures among which they are confounded. But 

 in Houna, the stratification is perfectly distinct; the 



