I 



BY LOFTCS HILLS, M B.E., M.SC. 133 



form of a limited number of composite batholiths. It, more- 

 over, seems possible that there finally resulted one huge 

 batholith underlying the greater part of Tasmania, the cupo- 

 las and satellitic injections from which now represent the 

 apparently isolated massifs as we at present see them. 



In spite of the great amount of work that has been ac- 

 complished, and the numerous descriptions written in regard 

 to the petrography of the Tasmanian diabase, yet, as pointed 

 out by Osann, there is very little in existence descriptive 

 of its field occurrence and structural relationships. We know 

 that it is intrusive and that undoubted sills and dykes occur, 

 and this was the state of our knowledge in 1902 with the 

 addition that other masses had had a laccolithic structure 

 suggested for them. It is therefore disappointing to have 

 to announce that up to the beginning of the past year no 

 material advance had been made in this connection. Cer- 

 tainly L. K. Ward recognised two distinct horizons of intru- 

 sive sheets near the King William Range, but no work was 

 done on the larger diabase massifs to elucidate their mor- 

 phology and mode of origin. During the past year, how- 

 ever, the geological surveys carried out in the Midlands by 

 P. B. Nye, on the East Coast by H. G. W. Keid, and during 

 the last few months by A. Mcintosh Reid, have supplied 

 valuable data which, along with that being acquired at the 

 present time in the extensions of those surveys, will prob- 

 ably enable a very complete summary of the field occurrence 

 of our diabase to be prepared. The evidence so far obtained 

 points to our larger diabase massifs being asymmetric lac- 

 colithic intrusions possessing an alm.ost vertical face on one 

 or more sides, but grading off into an intrusive sheet on one 

 or more of the others. 



Very little work has been done on the field occurrences 

 of the Cygnet Alkaline series and the probably associated 

 nepheline and melilile basalts, and the status of our know- 

 ledge in this connection is practically as it was in 1902. 



Beyond the areal mapping of some of ou;: olivine basalt 

 areas no advance has been made as to the mode of origin. 

 Certainly the negative evidence provided by the failure to 

 locate a single, volcanic cone is valuable but not conclusive 

 evidence of fissure eruptions. 



This short review of the progress in petrologic science 

 cannot be complete without a reference to that most interest- 

 ing discovery — the Darwin Glass. The credit of first bring- 

 ing this substance under the notice of the Geological Survey 

 belongs to Hartwell Conder, M.A., who in 1912, while acting 



