DIALLAGE ROCK. 645 



It ought, of course, to be introduced into the 

 tabular arrangement among the stratified rocks 

 of the primary class; and, as it occurs, indiffer- 

 ently, in company with gneiss, micaceous schist, 

 chlorite schist, and argillaceous schist, while it is 

 at the same time rare, it may conveniently be 

 placed in the table immediately before limestone. 



It abounds in the islands of Unst, Balta, and 

 Fetlar ; and occurs also, but in very small quan- 

 tity, at the northern extremity of the Mainland 

 of Shetland. It appears further to exist, in a 

 very limited manner, in Ayrshire and in Corn- 

 wall ; and I formerly remarked that it was said 

 to be abundant in Piedmont and in Corsica. 



In Shetland, the largest mass of this rock, 

 which is that of Unst, succeeds, in some places, 

 to gneiss ; in others, to micaceous schist, chlorite 

 schist, and argillaceous schist ; and it is also 

 found, both in this island and in Fetlar, in contact 

 with serpentine. 



Although the stratification is often very ob- 

 scure, it may be determined without much diffi- 

 culty ; partly by the conformable direction of 

 the masses to the prevailing bearing of the ac- 



