9i :me. c. e. tilley ox the GEAXiTE-cyEissEs [a'oI. Ixxvii, 



represents a rearrangement of the biotite-molecule must be con- 

 sidered. 



In the followmg equation 



(1) H,K,Mg,Fe.Al,.(SiOj9-i-3Si02;^2KAlSi30^-f2re,MgAl,(SiOj3-f2H20 



the formation of garnet and orthoclaseis represented as the product 

 of a reversible reaction, of which quartz and biotite are the inter- 

 acting members. Xow, this reaction is an important one that 

 is revealed in metamorphism. Under a high-grade metamorphism, 

 as has been shown elsewhere, garnet and orthoclase are prominent 

 constituents of the paragneisses of Sleaford Bar, and the revers- 

 ibility of the above equation is confirmed by the visible mineral 

 associations which these rocks reveal, where, owing to the limited 

 amplitude of diffusion, the equation is susceptible of confirmation. 

 Vnder diminishing pressures and lower temperatures, the reaction 

 proceeds in the reverse direction. 



It is probable that this equation, or one in all essentials identical 

 therewith, may represent one of the equilibria tending towards 

 establishment in the liquid magma. Garnet is characteristically a 

 high-pressure mineral, and the formation of the garnet-molecule 

 in the magma may be a function of the pressure. This reversible 

 equation might, therefore, not be established in magmas subject to 

 normal pressures during intrusion, to any degree. AVith higher 

 pressures and a consequent shift of the equilibrium strongly from 

 left to right, and under these conditions favouring the formation 

 of the garnet-molecule, its precipitation is arrived at with satura- 

 tion, and the removal of solid garnet from the solution involves the 

 reaction proceeding continuously to the right either until one of 

 the constituents is exhausted, or until some change of condition 

 disturbs the equilibrium. 



The presence of much volatile mineralizer as water, as X. L. 

 Bo wen '^ has indicated, probably involves a splitting-up of the poly- 

 silicate molecule of orthoclase. a reaction analogous to hydrolysis, 

 into the orthosilicate molecule and silica, the orthosilicate molecule 

 representing a constituent of the biotite molecule 



(2) KAlSi^O jKAlSiO^-f2Si02. 



Inspection of equation (1) shows that a comparative deficiency 

 of water-vapour favours the reaction proceeding from left to right, 

 and this is probably a fmlher factor to be considered, as noted 

 below. 



The presence of diopside and hypersthene as constituents of 

 acid granites is sufficiently familiar in the well-known charnockite 

 type, and their presence in the members of the Flinders Series 

 present no extraordinary points of interest. It has been remarked 

 that the existence of pyroxenes in rocks so far differentiated as 

 acid iiTanites implies a relative deticiency of volatile mineralizers, 



^ Joiu-n. G-eol. Chicago, vol. xsiii (1915) Supplem. pr,. 44-45. 



