BY W. H. TWELVETREES AND W. F. PETTERD. 45- 



in which there are rather peculiar nests of albite crystals- 

 The keratophyre here is rich in bright green chlorite. On 

 the Tyndal Creek there is a rather fresh keratophyre, with 

 a good deal of chlorite. On the ti'ack to Mount Tyndal the 

 felsite is much epidotised ; some of the Carlsbad tw ins have 

 one half replaced by epidote, and chlorite is developed 

 abundantly. The felspars of the keratophyre in the Ring 

 River adit have been replaced by aggregates of secondary 

 albite and quartz : the crystals of felspar here have sectional 

 fields \ev\ characteristic of keratophyres. There is a kerato- 

 phyre on the Crown Hercules overlying the ore body, with, 

 slate on both sides. At the Mount Black mine the shaft is 

 in keratophyre, identical with that of Mount Read. The 

 porphyritic felspars are here, too, surrounded by secondary 

 felspathic growth. Further north, on the Tasmanian Copper- 

 Company's property, there is a band of felsite in the lower 

 adit 40 feet wide, in which the few felspars are broken up 

 into aggregates of secondary albite. With reference to the 

 groundmass of these rocks. Professor Rosenbusch writes to- 

 ns : — '' Nothing is left of the original groundmass : it has 

 been converted into sericite, quartz, and albite. The newly- 

 formed albite felspar can be distinguished quite easily from 

 the older phenocrysts. The chlorite indicates original 

 pyroxene rather than biotite." 



Summarising the above, we have here the characteristics 

 of felsite and quartz felsite, and especially of keratophyre 

 and quartz-keratophyre. The rocks have a compact quartzo- 

 felspathic (felsitic) groundmass, with quartz and orthoclase 

 and albite phenocrysts, sometimes distributed sparingly, at 

 other times so crowded as almost to lose the porphyritic 

 stamp. In the typically porphyritic varieties are altered 

 spherulites and signs of flow structure. In a word, these 

 are ancient, now devitrified, lavas of the alkali -granite and 

 alkali-syenite families. The quartz keratophyres are the 

 granite volcanics : the keratophyres are of syenitic nature. 



There are no plutonic masses in the neighbourhood with 

 which we can connect this series of lavas-. The syenite, 

 which occurs in boulders north of Rosebery, has not been 

 found in situ : and the granite at Heemskirk is of younger 

 date. There is some pegmatitic granite on Jolly's section 

 at Lalfe Dora, which microscopically appears fresh and 

 post-Silurian. The mass or dyke of syenite porphyry at 

 Lynchford is evidently of great age. The mechanical 

 deformation of its crystals indicates that it was subjected 

 to the same earth movements as the slates ; and there is 

 consequently a likelihood of it being quite as old as the 



