770 Geological Society : — 



cent, of labradorite-phenocrysts, in a crystalline groundmass made 

 up of a more acid plagioclase, hypersthene, dialiage, and titano- 

 magnetite, with small quantities of biotite, and very little spinel, 

 apatite, and pyrite. A little secondary hornblende and some garnet 

 are present, but few other secondary products. Olivine is con- 

 spicuously absent. 



Theplagioclase-phenocrysts are more acid in their outer zones, and 

 the groundmass plagioclase is still more acid as determined optically 

 and by specific gravity and analysis. Analyses are tabulated of the 

 bulk of the rock and the groundmass, and of the separated plagio- 

 clases and the magnetite. Prom these the relative proportions of 

 the constituents are calculated, and the formula of the felspars 

 determined ; also the titan omagnetite proves to be a mixture of 

 magnetite with ilmenite. 



The order of crystallization is found to be : — (1) Phenocryst 

 plagioclase ; (2) plagioclase with magnetite ; and (3) plagioclase, 

 magnetite, pyroxenes. Theplagioclase-phenocrysts started to form 

 at points from 8 to 18 centimetres apart, and when they had grown 

 to some size a second crop of smaller crystals grew at nearer points ; 

 the magnetite was much aggregated round the plagioclase-pheno- 

 crysts, growing together with the plagioclase of the groundmass ; 

 and finally the remaining magma crystallized. The order of crystal- 

 lization is found to be consonant with the physico-chemical laws 

 applying to the phase liquid-solid. Graphic representations are 

 given to illustrate the order of crystallization of a ternary system 

 of plagioclase, magnetite, and p}^roxene ; and it is shown that the 

 separation of such a system would yield first the formation of 

 plagioclase-phenocrysts, secondly the solidification of a magnetite- 

 plagioclase eutectic, and finally an eutectic of plagioclase, magnetite, 

 and pyroxene. In considering the equilibrium between the solid 

 and the liquid albite-anorthite phase, it is found that equilibrium 

 must have been maintained sufficiently long for the phenocrysts to 

 acquire a composition different from the first-formed crystals, but 

 that eventually the equilibrium broke down — a matter possibly 

 connected with the size of the phenocrysts in relation to the 

 viscosity of the solution. The history of events in connexion with 

 the pyroxenes was probably similar to that in the case of the 

 plagioclases, but this portion of the subject is not fully worked out. 

 The temperature-interval of crystallization is estimated to have been 

 between about 1400° and 1000°. 



The author considers that this investigation establishes that the 

 processes of crystallization in a magma may be explained in all 

 details according to physico-chemical laws. 



2. ' On the Genus Looconema, with Descriptions of New Proterozoic 

 Species.' By Mrs. Jane Longstaff (nee Donald), F.L.S. 



