268 Geological Society. 



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 iu 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 

 Bare Elements.' By Baron A. E. Nordenskiold, E.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, andyttria 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 mineral 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 

 mineralogi.cal 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, 

 argon, or helium are referred to ; and under the head of minerals 

 bearing tantalum, mention is made of Giesecke's discoveries in Green- 

 land. Among these is fergusonite, one of the richest sources hitherto 

 known for obtaining that mysterious gas, or mixture of gases, which 

 on our planet seems to be almost exclusively confined to minerals 

 containing rare earths. ' The group of earths, as well as the group 

 of gases, of which we are here speaking, might, therefore, be com- 

 pared with certain genera among organic beings, whose species, 

 having not yet fully differentiated, offer to the descriptive zoologist 

 or botanist difficulties analogous to those with which chemists meet 

 in endeavouring to separate the rare earths and rare gases.' 



