VOLCANIC NECK AT THE BASIN, NEPEAN RIVER. 131 



admixture of the two, with the production of a syntectic 

 melt, which crystallised as pyroxene and pleonaste grano- 

 phyrically intergrown. In connection with the Basin rocks 

 it must be remembered that felspar crystallised late. 



Below is placed a chemical analysis of a troctolite from 

 the Basin (A) and with it (B) an analysis of a troctolite 

 from Coverack. 





(A) 



■(B) 



No 



RMS. 





Si0 2 



46-71 



45-73 





(A) 



(B) 



A1 2 3 



24-08 



22-10 



Orthoclase 



•56 



1-67 



Fe 2 3 



•51 



•71 



Albite 



20-96 



17-13 



FeO 



4-99 



3-51 



Nepheline 



— 



2-13 



MgO 



2-12 



11-46 



Anorthite 



49-76 



48-09 



CaO 



10-01 



9-26 



Corundum 



1-63 



— 



Na 2 



2-51 



2-54 



Diopside 



— 



7-07 



K 2 



•10 



•34 



Hypersthene 



6-12 



— 



Ti0 3 



•26 



— 



Olivine 



18-00 



18-36 



H 2 + 



1-22 ) 

 •69 J 



4-38 



Ilmenite 



•61 



— 



H 2 0- 



Magnetite 



•70 



•93 



MnO- 



•27 



— 



Water 



1-91 



4-38 





100-47 



10003 



100-25 



99-76 



Classification for both rocks -i- II. 5. 4. 5. 



Magmatic Name — Hessose. 



(A) Troctolite from the Basin. Analyst, G.D.O. 



(B) Troctolite from Coverack, Cornwall. 1 Analyst, F. T. S. 



Houghton. 



It will be seen that these rocks are very similar in 

 chemical composition. The Basin rock is more felspathic 

 and the Coverack rock more olivinic. The calculation of 

 the plagioclase felspars gives the following results, assum- 

 ing that in the case of (B) there is available silica to form 

 the polysilicate albite with the disappearance of nepheline: 



1 (a) Geol. Mag., 1879, p. 504. (b) Q.J.G.S., p. 906. 



