PIIYSIOGKAPIIY OF TIIK AUSTRALIAX ALPS. 3Sl 



I believe is destined to play a most important part in the elucida- 

 tion of a very difficult question, and add considerably to the sum 

 total of our knowledge of the various chanfjes which were wrought 

 in the Silurian sedimentary rocks during tlie Devonian period. 



There can be no doubt as stated by Mr. Howitt, that at Eusay 

 the tirst igneous rocks that were forced into the sediments as 

 veins and masses were varieties of aplites. Following tliese the 

 rocks which are now the holo-crystalline quartz mica diorites ; 

 while the whole complex of rocks, metamorphosed sediments, veins, 

 and masses of aplite and quartz diorite, have been crossed by 

 dykes of quartz porphrite, still later by dykes of diabase, and 

 finally in Tertiary times by a few dykes of basalt. 



At Omeo, the granite masses, taken as a whole, are stated to 

 represent an intrusion of plutonic rocks of several consecutive 

 ages of the same period of plutonic invasion. At Day's Hill, of 

 which I give sketch,* there is a mass of eruptive rock, younger 

 than the main mass of granites, and Mr. Howitt remarks,! ^^i^^t 

 as the granites and granite dykes penetrated the schists and 

 metamorphosed them, so did the quartz-bearing and quartz-less 

 porphyries of the Frenchman's (Day's) Hill, penetrate in masses 

 and in dykes, both the granites and the schists, and react on 

 them. 



In summarising the general results of his work on the sedi- 

 mentary, metamorphic, and igneous rocks of Ensay (p. 59), Mr. 

 Howitt remarks that — " two kinds of metamorphism may be 

 distinguished — dynamic metamorphism, or the effects produced by 

 heat, resulting from vast movements within the earth's crust upon 

 the sediments and the mineralised waters included in them — and 

 contact metamorphism, or the effects produced on the sediments 

 by masses of intrusive igneous rocks." 



The movements which tilted, folded, and crushed the sediments 

 at the close of the Silurian period over an enormous region of the 

 Australian Alps metamorphosed them into argillites, and where 

 the movements were greatest into mica-schist and gneiss. 



Following these results of dynamical metamorphism, the more 

 or less altered sediments were invaded by molten masses which 

 reacted on certain areas producing contact schists. 



At Omeo it is remarked | that — "The regional schists were 

 probably phyllites and fine grained mica-schists, and by the further 

 action of the invading granites have been converted for some 

 distance from the contact into mica-schist, tourmaline-schist, and 

 forms of gneiss." Some additional remarks on the schists of the 

 Livingstone Creek valley are to be found in the author's " Report 

 on a Geological Traverse of the Mitta." § 



* A GHoln-fical Sketch Section Australian Alps. Trans. R. Soc, S, Australia, 

 t Notes on certain Metanior)ihic Kocks at Onieo. Trans. R Soc. Victoria, 1887. 

 I A. W. Howitt. Notes on certain Metamorphic atid Plutonic Kocks at Omeo. Trans. / 



R. Soc. Victoria. 



§ Reports, Mining- Registrars, Victoria, 1887. Appendix by J. Stirling. 





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