106 Dr. Mac Culloch oji the 



of gneiss, in others with micaceous schist, chlorite schist, 

 argillaceous schist, and serpentine. It may also be said to al- 

 ternate with every one of those rocks, while the large masses of 

 it also contain thin beds of most of these substances ; and, in 

 addition to these, similar portions of hornblende schist, talcose 

 schist, actinolite schist, and, more rarely, of serpentine. 



It must of course be introduced into the system of the primary 

 rocks among the stratified substances ; and as I have on other 

 occasions, shewn that the order of succession among these is 

 variable and uncertain, so there is no place to which diallage 

 rock can exclusively be referred. 



With respect to its transitions, I must remark that it appears 

 to pass into talcose and chlorite schists, as well as to contain 

 thin beds or laminse of those substances. In this case, the 

 essential mineral, diallage, appears to change its character, so 

 as gradually to pass into talc, or chlorite ; but as the other 

 ingredient remains unchanged, the resulting compounds are 

 feldspar and chlorite, or feldspar and talc. The apparent 

 passage into serpentine is less genuine, and it takes place 

 where the diallage rock approximates to that substance. In 

 this case, it is probable that the feldspar is excluded, and 

 a serpentine substituted in its place; the difficulty in de- 

 termining the exact point of change, arises from the dark 

 colour of the compact feldspar, and the hard nature of the 

 sterpentine ; so that in this state of minute admixture with an- 

 other mineral common to both, they are scarcely distinguish- 

 able. It must be recollected that this rock is as yet but very 

 imperfectly known, although it occurs abundantly in Piedmont 

 and in Corsica ; being, in the former country, associated with 

 serpentine as it is in Shetland. It is probable that when it shall 

 become better known, it will be necessary to add much to, and 

 to modify somewhat of, those particulars here contributed to* 

 wards its history. 



In its internal large structure, it presents, independently of 

 its stratification, some peculiarities already hinted at, from 

 which arises that disposition to an irregular fracture which so 

 generally obscures the divisions of the strata. It is, in all parts. 



