196 DIALLAGE ROCK. 



more accurate accounts than they have usually done of 

 rocks in general. Its theory must therefore remain ob- 

 scure. From my own investigations, it should bear ah 

 analogy to gneiss ; having been originally deposited, as 

 a stratum, from water, and subsequently mineralized by 

 heat. But if the facts respecting it, quoted in the sub- 

 sequent history of Serpentine, are authentic, it is, like 

 that, an occasionally venous rock, and might therefore 

 claim a similar place among the unstratified substances. 

 Thus, analogous to gneiss in one place, it should be 

 analogous to granite in another; while this double cha- 

 racter is perfectly consistent. Thus, further, while 

 nearly resembling hypersthene rock, it might be, like 

 many traps or porphyries, a member of both classes ; 

 and it is, in reality, a secondary as well as a primary 

 rock, if the facts to which I have alluded are true. 



Though, in Shetland, it confers no particular cha- 

 racter on the form of the land, it is easily recognised, 

 even at a distance, by the peculiar aspect of the surface. 

 Being, like serpentine, unfavourable to peat, and 

 mouldering with difficulty into soil, it protrudes every 

 where, with an effect resembling that which is pro- 

 duced by scattered blocks of granite, while the inter- 

 vals are distinguished by the greenness of the pasture. 

 Balta displays the character of this rock when exhi- 

 biting a bare surface : its precipices being peculiarly 

 rugged, without marks of stratification, and broken into 

 innumerable angular parts, by fissures in every possible 

 direction, but bearing no resemblance to granite, or 

 trap, or any other rock. 



