130 the progress of geological research in tasmania, 



( 2 ) . Petrogenesis. 



Viewing the igneous rocks of Tasmania from the broader 

 standpoint of petrology and accepting the most comprehen- 

 sive significance of that word, the questions of chemical 

 composition, structure of the igneous masses, and mode of 

 origin come up for consideration and investigation. 



In regard to chemical composition it must be at once 

 admitted that our progress has been practically nil. The 

 number of rock analyses carried out during the last 18 years 

 is practically negligible. It was hoped when the Geological 

 Survey Laboratory was established in 1914 that rock analysen 

 would be systematically carried out if only slowly, but it 

 has been found impossible up to date, owing to the timt^ 

 taken in routine assay work, to devote any time to rock 

 analysis. It is hoped, however, that the conditions will be 

 improved in the near future. The serious hindrance to pro- 

 gress that this lack of rock analyses imposes is so obvious 

 that no further comment is needed. 



The petrogenic problems in Tasmania are decidedly 

 complex, and it cannot be said that any near approach has 

 been made to their solution. It can, hov/ever, be claimed 

 that some progress has been made. 



In regard to the question of geologic age it cannot be 

 more definitely stated of the Porphyroid Igneous Complex 

 than that it belongs to the Cambro-Ordovician. Some of the 

 effusive and fragmental members of that complex are con- 

 temporaneous with the larger sedimentary series of that 

 system, while others undoubtedly succeeded the main sedi- 

 mentation. On the whole the evidence points to the fact 

 that the greater part of the igneous series belong to the 

 closing phases of the Cambro-Ordovician, although this has 

 by no means been completely demonstrated. It is clear, 

 however, that the acid plutonic members represent the end- 

 point of the Epi-Cambro-Ordovician orogenic disturbance, 

 as these show much less crushing than the other members 

 of the complex. 



It has been demonstrated by Ward, and confirmed by 

 Waterhouse and Mcintosh Reid, although originally suggest- 

 ed by Waller, that the basic and ultra-basic members of the 

 Epi-Silurian igneous series are slightly older than the acid 

 and sub-acid members — in other words, that the basic por- 

 tion of the magma appeared at the beginning of the petro- 

 genic cycle, while the acid portion followed shortly after- 

 wards. As previously pointed out in this review, this pet- 



