Reports and Proceedings — Qeological Society of London. 279 



with rhyoHtic and pumiceous breccias and tuffs, geyserites, and 

 sinters. The rocks exhibit spherulitic and perlitic textures, 

 fluxion, devitrification, and impregnation with silica. In one case 

 the obsidian, after solidification, appears to have undergone the 

 following changes : — (1) devitrification, with formation of spheru- 

 lites ; (2) increase in temperature sufficient to destroy the double 

 refraction of spherulites and the earlier formed felspars ; (3) the 

 decomposition of parts of the spherulites, causing them to assume 

 the characters of a cross-grained felsite. The rocks from Kotorua 

 exhibit the effect of solfataric action resulting in the production of 

 a high percentage of silica in the rock, and the development of 

 hyalite and isotropic opal silica or geyserite. In some cases these 

 rocks have had silica substituted for portions of the spherulites 

 which have been dissolved, the fibrous structure being preserved in 

 opal-silica, which, nevertheless, exhibits double refraction. 



In conclusion, the author suggests a comparison of certain ancient 

 rhyolites of Great Britain with those of New Zealand affected by 

 solfataric action. As to the causes which may convert a glassy into 

 a lithoidal rhyolite we still seem to lack information ; it is possible 

 that the action of steam may be instrumental in effecting such 

 a change, but this is probably only an occasional agent, and the 

 more general cause of such changes must be sought elsewhere. 



2. "On the Discovery and Occurrence of Minerals containing 

 Eare Elements." By Baron A. E. Nordenskiold, F.M.G.S. 



The first mineral referred to is scheelite, and the next cerite, which 

 contains no less than four rare metals. The incandescent light 

 produced when the latter mineral is fused with charcoal-powder was 

 first observed by Cronstedt in 1751. The discovery of glucina, 

 lithia, selenium, and yttria is next referred to. Minerals containing 

 yttria and oxides related to it were, at one time, thought to be almost 

 limited to certain pegmatite-veins running in a broad zone on both 

 sides of the 60th parallel of latitude. Latterly, fluocerite, orthite, 

 and gadolinite have been found in Dalecarlia ; and among these 

 minerals Benedicks discovered a silicate of yttrium containing 

 1*5 per cent, of nitrogen and helium. The author discovered kaino- 

 site, a silico-carbonate of yttrium and calcium, among minerals from 

 Hittero ; and the same minei'al was subsequently discovered in the 

 flucan, fissures, and drusy cavities at the Nordmarken mines. The 

 last-mentioned discovery and others related to it appear to suggest 

 that the mode of formation of fissure-minerals is not so unlike that of 

 the pegmatite- veins of the Primary rocks as is generally supposed. 



Thorium, discovered by Berzelius in 1829, was originally obtained 

 from the rich mineral locality of Langesund (called Brevig in 

 mineralogical literature), but it has since been recorded from other 

 localities, including Arendal and Finnish Lapland. It is now 

 obtained from the monazite-sand of rivers in the Brazils and South 

 Carolina. Thorite contains about '5 per cent, of inactive gas, 

 probably a mixture of nitrogen and helium ; but the latter element 

 was first obtained from the mineral cleveite, also containing thorium, 

 discovered by the author in 1877. Other minerals bearing nitrogen, 



