OLDER PEEIDOTITES OE SCOTLAND. V 371 



appropriate names by which to designate the several varieties. 

 The modifications of the ferriferous enstatites known as bronzite and 

 hypersthene (paulite) respectively, are, as I shall show hereafter, 

 alteration-products, and the structures which characterize them are 

 by no means rigidly confined to varieties of a particular chemical 

 composition. 



The unaltered crystals of the rhombic pyroxenes diiFer so strikingly 

 in their appearance and optical properties from those of the same 

 composition in an altered state, that it is misleading to designate 

 them by the same varietal name. Possibly the difiiculty may be 

 got over by following the example set by Streng in the case of 

 bastite, and calling the unaltered forms of the minerals bronzite 

 and hypersthene respectively proto-bronzite and proto-hypersthene. 

 Accepting also the ordinary convention, that the name bronzite 

 belongs to the less ferriferous, and the name hypersthene to the 

 most ferriferous varieties, we may then perhaps indicate the chief 

 types of the unaltered rhombic pyroxenes as follows : — 



(1) Non-ferriferous enstatite (enstatite proper) containing less 

 than 5 per cent, of ferrous oxide. A colourless or nearly colourless 

 mineral destitute of pleochroism. Hardness 5 to 5'5. Specific 

 gravity 3-1 to 3-2. 



(2) ferriferous enstatite (proto-bronzite). "With a percentage 

 of ferrous oxide ranging from 5 to 15. Pale coloured, with feeble 

 pleochroism. Hardness about b-b. Specific gravity 3*2 to 3*3. 



(3) Highly ferriferous enstatite (proto-hypersthene). With a 

 percentage of ferrous oxide ranging from 15 to 2b. Darker coloured, 

 with strong pleochroism. Hardness about 6. Specific gravity 3*3 

 to 3-4. 



(4) Excessively ferriferous enstatite (amblystegite). With a 

 percentage of ferrous oxide ranging from 25 to 35. Dark coloured, 

 with intense pleochroism (colours from deep red to vivid blue-green). 

 Hardness 6 to 7. Specific gravity 3'4 to 3*5. 



While the crystalline forms and the goniometric measurements 

 show no difi'erences in these several varieties, the positions of the 

 optic axes and other characters undergo great modifications ac- 

 cording to their chemical composition. 



The last named of these varieties of the enstatite group has not 

 received the attention which it deserves. I have recently found 

 that it occurs by no means rarely as a rock-constituent. Not only 

 is it found, both in. its unaltered and in its altered state, in the 

 gabbros of the Western Isles of Scotland, but it occurs in the Newer 

 Palaeozoic lavas of Porfarshire and in the diorites of western 

 Sutherland *. It is a widely distributed mineral, for I have found 

 it in rocks from Norway, Saxony, southern India, western Africa, 

 and the Solomon Islands, forming an important constituent of 

 gabbros, diorites, quartz-diorites, andesites, and certain interesting 

 ultra-basic rocks, while it appears to be almost always present, 



* [I am indebted to my friend Mr. Teall for calling my attention, since 

 the reading of this paper, to the rocks of this and several other localities in 

 which the mineral in question occurs.] 



