Diaiiage Rock of Shetland, 107 



often penetrated by laminee, almost invisibly thin, of talc, 

 chlorite, or mica ; in consequence of which the rock yields, on 

 the application of force, in numerous directions. Hence it is 

 scarcely possible to procure square or regular specimens from 

 the varieties which possess this character ; and hence also the 

 cliffs break in the irregular angular manner already described. 

 It is also much intermingled, on many occasions, with short ir- 

 regular veins or masses of the feldspar, which forms its chief, if 

 not its characteristic, ingredient ; and thus also its texture 

 often varies much even within very narrow limits. 



That texture is often confusedly crystalline, like that of gra- 

 nite ; the rock breaking in the same manner indifferently in any 

 direction, though commonly with great difficulty, on account 

 of its extreme toughness. This is more particularly the case in 

 the small grained varieties, and also in those in which compact 

 feldspar is an ingredient : the larger grained kinds, and those in 

 which the feldspar is platy, or of the common kind, are gene- 

 rally easy to break. 



But it is often fissile with considerable ease in one direction, 

 while it yields with difficulty in the other; the texture re- 

 sembling that of gneiss, or being imperfectly schistose. In 

 this case the fissility arises, as it does in gneiss, from a parallel 

 tendency in the crystals of the diaiiage. 



I may lastly remark, that independently of those changes of 

 the magnitude or proportions of the two ingredients of which 

 it is composed, it often contains those veins called con- 

 temporaneous, in which the two minerals are either intermixed 

 in a very distinct form, and in large portions or irregular crys- 

 stals ; or in which one of the constituent minerals alone exists 

 to the exclusion of the other. 



Diaiiage rock is essentially composed of feldspar and diai- 

 iage, but it also occasionally admits of quartz, of mica, of 

 talc, of chlorite, and actinolite. I am uncertain whether the 

 mixtures of diaiiage and serpentine should be ranked under it, 

 or whether they do not more properly belong to the varieties of 

 serpentine. 



The diaiiage varies much in the magnitude of the crystals. 



