J 28 THE PROGRESS OF GEOLOGICAL RESEARCH IN TASMANIA, 



VI. PETROLOGY. 



(1). Petrography. 



The knowledge gained during the period under review 

 of the petrography of the igneous rocks of Tasmania has 

 been considerable. This is not surprising when it is remem- 

 bered that the late W. H. Twelvetrees was one of the most 

 skilled petrographers of the Commonwealth, and the result 

 of his 18 years' labour, combined with the work carried out 

 by other officers of the Geological Survey, represents a dis- 

 tinct advance towards a complete description of our igneous 

 rocks. 



It may at the present time be confidently claimed that 

 the petrographic descriptions of our Epi-Silurian plutonic 

 acid and basic rocks by Twelvetrees, Ward, Waller, Water- 

 house, Mcintosh Reid, and Professor Benson represent an 

 approach to a complete knowledge of the character of the 

 numerous rock-types of this series. 



Similarly it is justifiable to claim that the descriptions 

 of the composition and microscopic structure of the diabase 

 which constitutes such a great proportion of Tasmania, pre- 

 sented at various intervals by Twelvetrees, Petterd, Ward, 

 Professor Benson, A. Osann (Frieberg), and F. P. Paul, 

 constitute an almost complete demonstration of the petro- 

 graphy of this rock. 



Completeness in petrographic descriptions also charac- 

 terises the investigations of our Tertiary basalts, whether 

 they be the normal olivine basalts, the limburgite, or the 

 trachydolerite of Table Cape and Stanley. It is to Twelve- 

 trees, Petterd, and Ward that we owe our detailed know- 

 ledge of these. 



Although considerable advance has been made in de- 

 ciphering the variations in rock-types in the Port Cygnet 

 alkaline series and the probably comagmatic nepheline 

 basalts of Bothwell, and the melilite basalt of Sandy Bay — 

 work in which Twelvetrees, F. P. Paul, and Professor Ben- 

 son have been most prominent — yet there remains a very 

 large amount of investigating to be done before anything 

 approaching the detailed character of our knowledge in re- 

 gard to the Epi-Silurian plutonics is attained. It is in such 

 a case as this that there is severely felt the handicap to pro- 

 gress which is occasioned by the elimination of the subject 

 of geology from the University curriculum, as the Port 

 Cygnet alkaline series, by reason of both their interest and 



