96 



Rods of apatite witli transverse jointing are numerous, and 

 there are one or two narrow laths of a dichroic mineral which 

 I suspect may be hornblende. 



The presence of glass in some of the picrites, their por- 

 phyritic character and their varying quantities of felspar, all 

 tend to show that their present classification is not final. 

 Manifestly the holocrystalliue gabbroid varieties ought not to 

 be ranked with porphyritic ones. This difference is accom- 

 panied by a difference in their geological occurrence, the one 

 division being phitonic, the other a lava. The occurrence at 

 Mt. Horror is probably a basic segregation in the Tertiary 

 basalt there. 



I do not detect much vitreous base, but there is a consider- 

 able quantity of a clear substance containing, besides apatite 

 and glassy belonites, minute colourless needles of what is 

 usually considered to be secondary hornblende (grammatite, 

 tremolite). This substance permeates the sections in all 

 directions, and gives in bladed aggregates the feeble re-action 

 on polarised light characteristic of hydrous silica. It clearly 

 has no original place in an ultra-basic rock. I cannot 

 establish for it a pseudomorphous character ; the impression 

 conveyed is that it veins the rock generally. It may be seen 

 too in some of the hand specimens as white veins. I suggest 

 that it is the hydrous ferro-magnesian silicate, serpentine, 

 formed by the hydration of the olivine, which, upon this 

 assumption, formed once a larger proportion of the rock. 



It is worthy of note that needles of tremolite have been 

 observed in the olivine pseudomorphs and serpentinous 

 substance of English picrites*, and Rosenbusch regards 

 similar cases of their occurrence as associated with the 

 alteration of olivine. I do not see any traces of the mutual 

 intersection of belonites in the serpentine, regarded by 

 Bonney as suggestive of enstatitef. 



From the above description the nature and affinities of the 

 Mt. Horror rock may be approximately gleaned. Its 

 texture is coarser than that of a dolerite, and by its constitu- 

 tion it is an augite-olivine-felspar rock. Its relations are 

 evidently with the olivine-basalts, while it is connected with 

 the ultra-basic rocks by its high sp. gr., indicating a low 

 Si O2 %, and by its excess of ferro-magnesian silicates. These 

 remarks may serve to direct attention to this instance of a 

 very basic kind of basalt in Tasmania and elicit the com- 

 munication of further occurrences. 



* Teall, Brit. Petrogr., p. 86. 

 i Q. J. Geol. Soc, 1883. p. 255. 



Note,— In plate for picrite and ausjite picritie rwud picritic basalt. 



