BY W. H. TWELVETREES AND W. F. PETTERD. 51 



a very fine magnetite cross is visible. This is an embryonic 

 crystal with two axes at right angles to each other, neatly 

 marked out by octahedral grains of magnetite growing end 

 to end and forming a cross of singular symmetry and 

 beauty. The iron ore is very plentiful in this variety, 

 forming skeletal crystals everywhere. It is, as a rufe, 

 abundant in the varieties which possess any interstitial 

 groundmass. In the holocrystalline descriptions it is 

 present in larger grains or crystals, but in very small 

 quantity, and from the ferriferous borders of many of the 

 augites, it is reasona1)le to suppose that the iron in the 

 rock has been largely utilised in that way. 



Quartz. — This is an unexpected mineral in rocks of 

 this class, but w^e have found it microscopically intergrown 

 with felspar (granophyric intergrowth), in a piece of dole- 

 rite from the top of Mount Faulkner, kindly furnished 

 by Mr. R. M. Johnston. Under such circumstances it 

 must be an original constituent. 



Mica. — A very little light brown biotite occurs in the 

 Launceston dolerite at the Cataract Gorge, and at the place 

 on the Elphin Road where the rock crops oift opposite Mr. 

 Thomas Corbett's grounds. The mineral is not associated 

 with any chloritic products, and appears to be original. 



Actinolite. — Needles of this mineral are to be found in 

 the rock at the Railway Station, Hobart, and this is the 

 only locality where we have observed it. It is rather 

 strange that no hornblende is noticeable in any of our 

 specimens, as it is not at all uncommon in the Swedish 

 Hunne diabase, which structm'allj^ resembles many of the 

 Tasmanian occurrences. 



Chlorite. — This substance is now universally admitted 

 to be only a secondary constituent of diabase, and to be of 

 no value in classification. Rosenbusch traces it to the 

 weathering of the augite mineral. Many Tasmanian 

 dolerites are perfectly fresh, but others contain a green 

 chloritic mineral between the felspars, and even in the 

 felspars themselves. In some the augite has been entirely 

 replaced by fibrous chlorite, and the rock would be called 

 diabase by many English petrographers. Still this chlori- 

 tised dolerite is not a separate geological unit, but forms 

 part of the mass of the fresh rock, and has doubtless received 

 its character from the purely local action of ordinary 

 meteoric agencies. This fact suggests the old question of 

 dolerite versus diabase. Dogmatism is inadmissible here. 

 The consensus of petrographical opinion must be allowed 

 to prevail. 



