78 



" FAYALITE BASALT. 



to be justifiable. The terms " Dolerite," " Anamesite," 

 and " Basalt " are here applied respectively to the coarsely- 

 textured, finely textured, but still visible to the naked eye, 

 and the very finely textured or microscopic varieties. 

 This rock, then, would preferably be classed as a basalt as 

 regards texture, though the fayalites are just visible to the 

 naked eye. Briefly, therefore, this rock is a basalt in 

 which fayalite has replaced augite and olivine. Macros- 

 copically, the rock is of a dark, compact appearance; 

 fracture, conchoidal. The fayalite crystals are just visible 

 as small dark red spots, which stand out clearly under an 

 ordinary hand lens. The specific gravity obtained by 

 weighing in air and water, of two specimens is 2'81. 

 According to Von Lasaulx the specific gravity of basalt 

 varies from 2-80 to 3*00. 



The following is a chemical analysis of an average 

 specimen of fragments taken from the upper, middle, 

 and lower zones of the flow, care being taken to avoid 

 weathered fragments : — 



... i7-21 

 ... 16-06 

 ... 11-87 



43 

 34 

 12 

 40 

 •51 

 •55 



Total... 



99-49 



From the nature of the case, that is on account of the 

 gradual merging of different varieties of rocks, it is difficult 

 to state definitely a typical average analysis of Basalt, but 

 most analyses show MgO in considerable quantity, say, 

 from Wo or three to seven or eight per cent., and in several 

 analyses of this rock only traces of MgO were obtained. 

 This would point to the olivine mineral being true Fayalite, 

 that is, FeO, SiOj. Not having material for Specific Gravity 

 solutions, we were unable to separate any of the fayalite 

 crystals and confirm this point, but trust, at some future 

 time, to investigate this matter, and bring the results of our 

 investigations before the Society. 



The following is a brief account of the microscopic 

 characters displayed in thin section by this rock : — The 

 most striking mineral under the microscope is Fayalite, 

 which appears in crystals and grains of a beautiful orange- 

 yellow colour in ordinary transmitted light. In thicker 



