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VARIETIES OF PYROXENE. 



limestone of Shinness, where tremolite also occurs, forming crystals 

 nearly a foot long. The magnesia lime amphibole, called edenite, 

 is found in the localities already alluded to Milltown, Glen Urqu- 

 hart, &c., which have yielded so many minerals in the granular 

 limestone. Actinolite, on the other hand, is characteristic of horn- 

 blendic gneiss, especially at Hillswick, in Shetland, and typical 

 hornblende has been found by Heddle on the north-west of Ben 

 Spinna in Durness, but hornblende is rather associated with the 

 diallage rocks of Shetland and the diorite of Portsoy. Like augite, 

 it undergoes the usual alterations by taking up water, peroxidation of 

 the iron, diminution of the lime, and partial removal of the silica, 

 all these changes being attributable to the action of waters passing 

 through the earth. 



By means of the following tables the varieties of Amphibole and 

 Pyroxene may be contrasted ; but beyond the fact that Amphibole 

 usually contains rather more magnesia and less lime, there is no 

 striking difference in composition ; and it would not be possible to 

 estimate the chemical composition which would give rise to one 

 variety rather than another if a sedimentary rock were metamorphosed. 

 The evolution of these minerals seems to depend upon the other 

 mineral constituents developed in the process of crystalline change. 



