124 



ME. C. E. TILLEY ON THE GEANITE-GXEISSES [vol. Ixxvii, 



may enter in varying proportions. A recent analysis, b}" Dr. H. 

 S. Washington,^ of the augite of the Stromboli basalt shows that 

 it consists of the following groupings : — 



CaMgFeSi,0, = 80-12 

 MgFeSiOg = 7-48 

 MgFeAl,SiO,= 7-03 

 NaFeSi.O, = 5-46 



This is perhaps a less aluminous type than is usual in rocks of 

 basaltic composition, as man}^ doleritic augites show an alumina 

 percentage reaching or exceeding 6 to 7. The entry of the mag- 

 nesium metasilicate molecule in large amount into the augite 



Fig. 11. — A pyroxene-garnet aggregate. X 125. 



[ah = secondary augite and hjrpersthene ; bi=biotite ; mg = magnetite.] 



constitution has been shown b}^ Wahl to be accompanied by a 

 marked change in the optical properties — notabh^ a decrease in the 

 optic axial angle, so that with a certain composition the pyroxene 

 becomes uniaxial. 



Among the ' relict ' pyroxenes of the Lincoln dolerites are types 

 with a low optic axial angle, some approaching uniaxiality ; and 

 the resemblance to members of the enstatite-augite group is f mother 

 manifest in the presence in some of these of a salite striation. 

 This influence of the MgSiOg molecule is to be suspected, when the 



^ Amer. Journ. Sci. ser. 4, vol. xlv (1918) p. 463. 



