Vol. 56.] ERUPTIVE ROCKS FROM NEW ZEALAND. 501 



Under the microscope the rock is seen to be aperlitic rhyolite, 

 pervaded by a small and rather ill-defined spherulitic structure. 

 There are also much larger spherulites sometimes forming aggre- 

 gates. The double refraction of these larger spherulites is seen, 

 when tested with teinte sensible No. 2, to be positive, but the cross 

 is often so broken up and distorted that it is then difficult to ascertain 

 their optical sign with any certainty. The character of the smaller 

 spherulites is still more difficult to recognize, but they also appear 

 to be positive. 



Cracked and corroded crystals and angular fragments of quartz 

 are plentiful in this section, and there are also some porphyritic 

 crystals of felspar : some of these being sanidine, others apparently 

 andesine, the former preponderating. Porphyritic crystals of biotite 

 also occur, often well-developed, but not very numerous. The green 

 colour of the rock is probably due to the fine dust and microlites 

 which pervade it. The latter extinguish parallel to their length, 

 and are probably epidote. 



Rocks from Rotorua. 



In the Ninth Annual Report of the United States Geological 

 Survey, 1887-88 [1889], there is a treatise on the * Formation of 

 Travertine & Siliceous Sinter by the Vegetation of Hot Springs ' by 

 W. H. Weed, in which, after dealing very fully with the subject 

 in its relation to the geysers and hot springs of the United States, 

 some details are added concerning his examination of specimens of 

 siliceous sinter from New Zealand. He states (op. cit. p. 673) that 



' The siliceous sinters from Rotorua vary from pulverulent deposits of impure 

 silica to dense, white opal sinters. Two of the specimens were evidently formed 

 about spouting vents, showing the peculiar structure and beaded surface pro- 

 duced by the evaporation of spattered drops of water. Such sinters, to which 

 the name of geyserite may be most properly applied, are very common about 

 the Yellowstone geysers, occurring often in beautiful coralloidal forms, some- 

 times possessing a bright pearly lustre. The New Zealand specimens are parts 

 of an old deposit formed in this way, and consist of numerous little pillars 

 formed of many convex layers of pink and white silica, resembling a pile of 

 minute caps, one upon another. This geyserite is wholly the result of evapora- 

 tion, which adds film after film of glassy silica to the surface of the deposit, 

 as often as wet by the steam or spray from the geyser.' 



Mr. Weed further states (op. cit. p. 674) that 



' Two of the specimens are of especial interest because their structure 

 indicates that the algous life of the hot waters of Rotorua produced siliceous 

 sinter.' 



It appears, however, from comparison of this statement with that 

 previously quoted, that the siliceous sinters of Rotorua can be formed 

 without organic aid as well as with it. That Mr. Weed also regards 

 the question of the deposition of these sinters from both points of 

 view, is seen when he proceeds to say (op. cit. p. 676) : 



'No information is obtainable relative to the comparative abundance of the 

 different types of sinter, but the presence of acid, and comparative scarcity of 

 alkaline waters shown by the list of springs published by Dr. Hector, leads to 



