193 



CHAP. XXXI. 



Diallage Rock. 



THIS remarkable compound is among the more rare 

 of the primary rocks, and is, therefore, not very well 

 known. Fortunately, our own country contains one 

 conspicuous tract of it, where its chief characters may 

 be examined, and whence the present description must 

 be derived ; since foreign writers have not given us 

 very satisfactory accounts of those which exist on the 

 continent. This occupies the whole of Balta, and 

 portions of Unst, Wya, and Fetlar, in Shetland. It 

 occurs also, in Piedmont, in Corsica, and at Crems in 

 Austria ; being further said to form the Zoltenbergin 

 Silesia, and to be found in Norway, even as far as the 

 North Cape. It is to be wished that the details of its 

 geological connexions, in all these situations, were 

 carefully studied and described. 



This rock is a compound of diallage and felspar. 

 It's varieties depend on the nature of the felspar, which 

 is either the common or the compact kind, on the 

 'varying proportions of the two ingredients, and on the 

 various-magnitude of the integrant parts. For these 

 and other details I must refer to the Classification of 

 Hocks ; the mixtures of serpentine and diallage being 

 ranked under the varieties of the former substance. 

 Independently of its stratification, Diallage rock pre- 

 sents some peculiarities, whence arises that disposition 

 to irregular fracture which so generally obscures the 

 divisions of the strata. It is often penetrated by thin 

 laminae of talc, chlorite, or mica ; in consequence of 

 which it yields in numerous directions. It is also often 

 intermingled with short irregular veins or masses of 

 the felspar which forms its chief ingredients ; and thus 



VOL. II O 



