296 H. S. Siiinmers : 



the iron oivx, is also a l)iniiiiL'ialie eoiiipound e-oiitaininir appi'oxi- 

 mately 41 per (.-eiit. of aiiipliil)olt' or pyroxene and 59 pei- cent, of 

 anorthite. 



Barker's generalisation as to the relationship of Ixaiidies of 

 igneous rocks to tectonic movements is criticised, and evideme is 

 advanced to show that in its present form it fails when applied to 

 Victoria. 



The origin of igneous rocks is briefly dealt with, and it is; sug- 

 gested that the primary differentiation of a parent magma is into 

 two portions, a basic and an acidic. The basic poi'tion gives rise 

 to the basalts, gabbros, norites, etc., jand the acid or dacitic magma 

 by serial differentiation produces andesites, rhyolites. granites, etc. 

 Other rocks are subordinate in amount and are believed to be due 

 to subsidiary differentiation of a partial magma from the basaltic 

 or dacitic magma when the molecular balance of the latter ha^* 

 been upset by assimilation of foreign material or by incomplete 

 primary differentiation. 



VIM. References. 



1. Coleman. A. P. The Sudbury Laccolithic Sheet. Journ. 



Geol.. Vol. XV.. Pt. II.. 11)07, pp. 75!)-7?<2. 



2. Daly. K. A. Magmatic Differentiation in Hawaii. Journ. 



Geol.. Vol. XIX.. No. 4. 1!)11. pp. 289-31G. 



3. Gregory. J. W. Tlie Heathcotian. a Pre-Ordovician Series 



and its distribution in Victoria. Proc. Hoy. Soc. Vic. Vol. 

 XV. (n.s.). Pt. II.. 190:5. 



4. Harker. A. The Natural History of Igneous Rocks. London. 



1909. 



5. Harker. A. Presidential Address— Section C— Brit. Ass. for 



Adv. of Science. Portsmouth. 1911. pp. :{70-:}81. 



6. Harker. A. The Teitiary Igneitus Hocks of Skye. Mem. of 



Geol. Surv. of Unit. Kingdom. 1904. 



7. Howitt. A. W. Notes on the Rocks occurring between Lime- 



stone Uiver and .Mount Leinster. Iiep. .Mines Dep. Vic, 



1H90. pp. ;5o-;5;}. 



8. Howitt. A. W. Notes on Diabase and Adjacent Fitrmations 



of the Heathcote District. Special l{ep. Dej.t. of .Mines Vic. 

 1896. 



9. Howitt. A. W. Various papers on Gippsland. Q.J.G.S.. 



Proc. Hoy. Soc Vic. .Mines Dept. Publications. 



