00 



The rock, as a whole, consists of siliceous spherules, some- 

 what smaller than peas, in a closely packed mass, occasionally 

 separated by tracts of quartzose, or quartzo-felspathic 

 (felsitic) ground mass. Viewed with the naked eye, these 

 spherules are grey or pale green in colour, or sometimes 

 stained red, which gives the stone an attractive appearance. 

 A quantity of porcellanic alumina silicate (resembling por- 

 cellanite, but very siliceous) surrounds many of the spherulites, 

 and is probably derived from the ingredients of the ground 

 mass. 



Microscopically, the spherulites show a radiation hardly 

 recognisable in thin section by ordinary light, but plain enough 

 under cross prisms, when the fixed black cross declares them 

 to be true spherulites. Their central point is often indis- 

 tinguishable, or it consists of one or two quartz grains, which 

 have been the nuclei of the segregation. Where the 

 spherulites are closely set together, they are united by 

 sutural lines, interlocking and mutually interfering with 

 one another. Their margins are always well defined, aud 

 usually somewhat crenulate. Wherever they are surrounded 

 by the ground mass, they are adorned by a marginal fringe of 

 felspar fibres. The Hues of radiation are wavy, and the general 

 surface appearance of the microscopic section is puckered or 

 crumpled. The ground mass presents, under polarised light, 

 the speckled field of a felsitic substauce, and occasionally 

 show r s a tendency to develop minute spherulites. 



From the preceding there appears to be little reason to 

 doubt that this rock is an unusually fine example of spheru- 

 litic felsite. Figures of somewhat similar spherulites will 

 be found in the Quarterly Journal of the Geological Society, 

 vol. xxxix., 1883, pi. x., illustrating spherulitic quartz- 

 porphyry from. St. David's, discussed by Sir A. Geikie in his 

 paper " On the supposed Pre-Cambrian Rocks of St. David's." 

 The ground mass in that rock, however, is entirely micro- 

 crystalline, while in the Tasmanian one it is felsitic. Many 

 of the British spherulitic felsites are devitrified rhyolites ; 

 whether the Tasmanian one is rhyolitic or not must remain 

 an open question at present, though the absence of known 

 rhyolites here favours the reference of the Zeehan boulder 

 to a granitic source. Its parent rock was probably connected 

 with some granite protrusion on the West Coast. Its specific 

 gravity is 2"63. 



Castray River. — A similar rock has been found as a 

 boulder in the bed of the Castray River. The spherulites 

 are of the same size as those in the Zeehan boulder, and are 

 much the same in appearance. They interlock along their 

 crenulated margins, and are often pressed into a more or less 

 polygonal form. When elongated, the centre of the dark 



