BY W. H. TWELTETREES AND W. F. PETTERD. 55' 



retains "diabase" for massive sills, dykes, laccolites, &c.,. 

 and applies dolerite to the less important intrusions of 

 similar rocks. Rosenbusch states that diabase, when 

 fresh, is undistinguishable from tholeites and dolerites : 

 Professor Lacroix, in France, upholds the term dia- 

 base as the group name. Looking at the present pi-ac- 

 tice of i)etrographers all over the world, we see that 

 there is a preponderance of agreement in favour of the 

 term " diabase." The future will show whether English 

 petrologists will agree to surrender their somewhat isolated 

 though thoroughly logical use of "dolerite," and fall 

 into line with their colleagues in other countries. All that 

 can be done at present is to observe the trend of petro- 

 logical opinion, and if one or other of the terms becomes 

 obsolete, it will be necessary to adopt the one which gains- 

 general recognition. Meantime we venture to adhere 

 to the English practice, and call this rock dolerite in its 

 fresh condition and diabase in its chloritised state. 



The tertiary basalts of Circular Head, Table G&pe,. 

 Lefi'oy, &c., often exhibit a coarse intersertal or slightly 

 ophitic structure, and would correspond with what goes 

 under the name of dolerite in Germany. In point of 

 coarseness such basalts are dolerites. Thej' nvAj be dis- 

 tinguished from our mesozoic dolerite by their abundant 

 olivine, glassy base, and greater freshness. 



The discovery of a little auriferous wash in the first and 

 second basins of the South Esk at Launceston has led a, 

 few people to believe that the mesozoic dolerite might be 

 gold-bearing ; but the fact is that the sand which was 

 obtained contained, besides small flakes and water-worn 

 pellets of gold, grains and crystals of quartz, zircon^ 

 sapphire, and ilmanite, all minerals of the granite and 

 slate country in the upper reaches of the river, and must 

 be referred to that source. No useful minerals have yet 

 been found in this rock, and the lodes and reefs of our 

 Tarious mines are all of earlier date. The experience of 

 our miners in this respect has been so uniform that search 

 for ore deposits in the dolerite is invariably regarded as 

 useless. 



