390 SKY. GEOLOGT. HYPERSTHENE ROCK. 



its real nature. These several varieties are all irregularly 

 mixed, occurring in close contact in the same continuous 

 mass, occasionally passing into each other, and often dis- 

 posed in such a tortuous manner as would arise from 

 disturbing a mixture of different tenacious fluids. One 

 rare variety yet remains, especially deserving of notice. 

 In this the crystals of hypersthene are in small compressed 

 prisms, disposed with their flat surfaces parallel to each 

 other and to the general plane of the mass, as mica is 

 in gneiss, and producing the same fissile tendency. Like 

 the mica in the trap of Kerrera and Seil, it may be sup- 

 posed to throw doubts on the opinion that the fissile 

 property of gneiss, or even of micaceous schist, is the 

 result of mechanical disposition. 



As this rock has never hitherto been observed, or else 

 has been confounded with the common varieties of trap, 

 it will be useful to mineralogists to know that it also 

 occurs at Airdnamurchan, and that it forms the supposed 

 greenstone of that promontory. In this place also it 

 contains those veins in which the large concretions of 

 hypersthene are found. It is probable that the rock 

 which produces the same mineral in Labrador is of a 

 similar composition ; nor is it unlikely that it will here- 

 after be found in many places where it has yet been 

 unnoticed, or, as in Airdnamurchan, confounded with 

 other substances. 



The hypersthene rock of the Cuchullin is traversed by 

 veins of different kinds, which it will be more useful to 

 describe here than in the section to which the considera- 

 tion of trap veins is referred. 



The first variety of these to be noticed, contains the 

 same ingredients as the including rock, with some occa- 

 sional additions; but the hypersthene always forms the 

 most conspicuous part. They are generally thin and not 

 persistent, and at the same time pass insensibly into the 

 surrounding mass. 



But the most numerous class consists of a blueish 

 grey compact felspar approaching to clay stone, very 



