522 W. N. BENSON. 



syntectic, pleonaste, enstatite and diopside are produced. 

 Pleonaste separates at once, forming fine fibres standing 

 perpendicularly to the boundary of the solution-spaces, or 

 irregularly shaped blebs in the melt, which crystallises as 

 pyroxene. Making the simplifying assumption that the 

 felspar is anorthite this reaction can be expressed in the 

 following formula : — 



m CaO, A1 2 3 , 28iO a + n 2(Mg. Fe)0, Si0 2 = 

 Anorthite. Olivine. 



(m - 1)J CaO, A1 2 3 , 2Si0 3 } Anorthite 

 + A1 2 3 , (Mg, Fe)0. Pleonaste 



+ CaO, (Mg,Fe)0, 2 SiO.,. Diopside 

 + 2{(Mg,Fe)0, SiO B }. Enstatite 

 + (w-2)j2(Mg,Fe)0, SiO,}. Olivine 

 That the spinel rocks do not always contain spinel, enstatite 

 and diopside, in just the proportion indicated by the formula 

 may be due to the original presence of pyroxene in the 

 felspathie or peridotitic magma, probably the latter. So 

 far, however, I have been unable to distinguish between 

 primary and secondary pyroxene in the microscopic examin- 

 ation. Another method of production of pyroxene more 

 active in the more felspathie rocks, and apparently sub- 

 ordinate in the femic rocks may be expressed in the follow- 

 ing formula: — 

 CaO, A1 2 3 , 2SiO„ + 2(Mg, Fe)0, 1 



Anorthite. Olivine. Augite. 



The assumption here made that the felspar has crystallised 

 first is not without foundation. Pratt and Lewis 1 adduce 

 evidence to show that in a basic magnesian magma the 

 aluminous minerals crystallise first, quoting Williams obser- 

 vations on the felspathie peridotites of Maryland. 2 Harker 



* Corundum and the lVn-1-tit.-, ,,f W..-:,.in North Carolina, North 



