MetamorpliisTYi of South Australian Dolomites. 449 



from what the writer calls the high plateau of south-east Brazil. 

 This ,'ould indicate that the diamonds now found in the Lavras series 

 in Bahia had their origin in an igneous rock somewhere in the high 

 plateau to the south. Carrying the inference still further we might 

 assume that the sudden appearance of the diamonds in the Lavras 

 series and their absence from earlier strata (which seems to be the 

 summary of the evidence of Derby, Branner, and Crandell) would 

 indicate the formation of the diamond in an igneous rock of later 

 Paraguassu age. The weathering of an intrusive rock of this period 

 would then give rise to the diamond-bearing Lavras series. 



The quartzites of the Diamantina region in Minas Geraes are 

 supposed to be contemporaneous with the Jacobina series of Bahia. 

 The overlying strata have been removed by denudation. If the 

 assumed pipes of Sopa, Boa Vista, and elsewhere are of late 

 Paraguassu age there must have been considerable erosion with 

 removal of their diamond contents. This would certainly account 

 for the rich deposits in sedimentary rocks lying to the north. The 

 present sections are the deep-seated remnants of pipes which no 

 doubt towered above their present sites for many hundreds or even 

 thousands of feet. 



All the above-mentioned rocks, with the exception of group 9, 

 are of early Palaeozoic age, and have been highly metamorphosed. 

 Whether the sericitic schist of Sopa is the metamorphosed remnant 

 of an igneous intrusion, and, if so, by what chemical process it has 

 been so metamorphosed is a question for the geological chemist. 

 The commercial aspect of the question, the discovery of the original 

 home of the diamond which has scattered its wealth over such great 

 areas of sedimentary rock in Brazil, would certainly warrant a closer 

 investigation into the subject. 



The Metamorphism of the Pre-Cambrian Dolomites of 

 Southern Eyre Peninsula, South Australia. 



By C. E. TiLLEY, B.Sc, A.I.C., Emmanuel College, Cambridge. 



(PLATE IX.) 



Introduction. 



The Hutchison Series 



Dolomites of the Hutchison Series. 



(a) The Sleaford Bay Section. 



(6) Dolomites of the Hutchison Area. 

 Chemical Changes involved in Metamorphism. 

 The Calc-magnesian Silicate Rocks of the Hutchison Series. 



(a) Diopside Rocks. 



(6) Diopside Microcline Rocks. 

 The Chemistry of the Development of the Calc-magnesian Silicate Rocks- 

 Conclusion. 



Introduction. 



rpHE basement platform of Southern Eyre Peninsula consists 

 -*- of a series of igneous and metamorphic rocks of pre-Cambrian 

 age. Rising from beneath the Mesozoic and '^ertiary strata of the 



VOL. LVII. — NO. X. 29 



