270 Transactions. — Geology. 



ance of granulation. The ground-mass is crossed by straight 

 streaks and spots of a dark-greenish colour, which under the 

 fifth objective are resolved into dark-green specks, lying in a 

 yellowish-green base. I could detect no signs of dichroism in 

 this yellow-green mineral. Wheu the nicols are crossed, the 

 ground-mass breaks up iuto a very fine mosaic of dark grey and 

 yellowish white, the milky portions behaving like the granular 

 portions. This mosaic consists of clusters of similarly oriented 

 granules which extinguish simultaneously. When the axes of 

 the nicols are placed at an augle of 45° most of the mosaic dis- 

 appears, but as the stage is revolved some portions take a blue 

 colour. When these portions are viewed with crossed nicols 

 their colour is brownish-yellow. 



From this it will be seen that the Mataura rock is very like 

 the felstone from North Wales described by Mr. Clifton Ward 

 in the " Quar. Jour. Geol. Soc." vol. xxxi., p. 400, and those 

 from Shropshire described by Mr. Allport in the same journal, 

 vol. xxxiii., p. 454. 



In sections that are nearly equatorial the spherulites always 

 show, both in ordinary light and in crossed nicols, a radiating 

 structure from the centre through the dark ring. In more tan- 

 gential sections this structure of course becomes obscure. The 

 radiating fibres stop abruptly at the junction of the ring with the 

 granular ground-mass, and there is no appearance of an inter- 

 mediate hyaline zone. But where two spherulites meet, rather 

 large grains of quartz have sometimes been developed, and these 

 quartz grains are also occasionally seen in the spherulites them- 

 selves, either in the centre or in the ring. The long green 

 streaks, mentioned as occurring in the ground-mass, run into or 

 even quite through the spherulites without disturbance. 



Another specimen of the same kind of rock has numerous 

 small spherulites without any ring scattered through the base. 

 These spherulites show a fixed interference cross, and the rock 

 passes in places into a laminated felstone. 



2. Perlitic felstone. — This is a green rock, weathering paler, 

 with veins and nodules of chalcedony, and a hand specimen 

 might easily pass for chert. Its specific gravity is 2*50. Under 

 the microscope, in ordinary light, the ground-mass is a colour- 

 less glass with numbers of dark chrome-green specks. These 

 sp<cks are gathered thickly together in curved bands, which 

 imitate exactly the perlitic structure of many vitreous rhyolites. 

 Between crossed nicols the ground-mass is anisotropic, but it 

 shows no mosaic. The chalcedony calls for no special remarks. 



3. Laminated felstone. — There are several varieties of these. 

 In some the larnime are pink and white, in others grey and 

 white. In thickness the laininae vary from very minute up to 

 - 05 inch. Sometimes they are approximately parallel; in 



