IGNEOUS ROCKS OF TASMANIA. 295 
in Wales. These are the forms which in Germany were 
originally called porphyroids and flaserporphyries.’’* 
An analysis of this rock from exposures on the North 
Hercules section was made by Mr. W. F. Ward, Government 
Analyst of Tasmania, with che following result : — 
Per cent. 
SiliGa tease wt ees tite tian ements POL Gee ame 
NC GAIT ae eee rane Sa TD) 
Oxidetotsiromen seek cee ene was 2-25 
Epes Reece Be MRL ere Sirs ae tee 2-00 
Maomnesta 25) seiateas egkece Oc oy 60% 0-60 
Potash: ey sce patie oie ae 2-04 
Soda... et ye er ase as | 3:48 
Ihoss at “red heate sees eee 120) 
100 - 00 
It is not high in Na© for a keratophyre, but not low 
enough to exclude it, while che excess of soda over potash is 
very distinctive. A few more analyses are desirable to place 
the determination upon a thoroughly satisfactory basis. 
All round the arm of the Huon called Port Cygnet, and 
crossing it in a N.E.-S.W. direction, is a belt of alkali rocks, 
which are developed as elzolite-syenite, alkali-syenite, tin- 
guaite, and sdlvsbergite, phonolite, or trachyte, contaiming 
nepheline, haiiyne, egirine, analeime, and sodic augite, 
melanite gravel, with all the accessory minerals of the 
nepheline group of rocks. The zone extends north to Oyster 
Cove, and to an undefined distance on the south side of 
the River Huon. The sediments into which some of these 
eruptive rocks have intruded are Permo-Carboniferous ; 
some of the trachytes are apparently contemporaneous. As 
the axis of the peninsula between Port Cygnet and D’Entre- 
casteaux Channel is crowned with Mesozoic diabase, the rela- 
tions of the latter to the nepheline belt will be a crucial test 
of age, as far as the Tertiary period is concerned. The dis 
trict is economically interesting from a good deal of alluvial 
gold having been shed into the flat at Lymington from the 
trachytic rocks, apparently from their contact-line with the 
sedimentary strata. The rocks at the contact are silicified, 
and carry minute quantities of gold (and silver up to 6 ozs. 
per ton). The search for fissure lodes as sources for the 
gold has been futile. 
* Felsites and Associated Rocks of Mount Read and vicinity, 
W.H.T. and W. F. Petterd. Proc. Roy. Soc. Tas., 1898-9. 
