Reviews — Museum Htimfrediamim. 379 



marked bed, which I photographed and reproduced in the paper 

 just referred to. 



I was therefore justified in pronouncing against the likelihood of 

 finding quartz-reefs in this neighbourhood, on the grounds that the 

 experience of prospectors was against it, and that from general 

 mineralogical and geological considerations it was impossible to 

 get gold-bearing reefs there. 



How, then, can it be explained that the quartz-reefs do contain 

 gold on the surface, sometimes in such quantities that the volume of 

 the gold is far greater than that of the quartz, and the gold nuggets 

 are so fresh, some of them, that crystalline forms are present ? It is 

 impossible that the latter can have been rolled any distance. 



The theory of solution by ferric sulphate, I think, explains all the 

 facts here as well as in Knysna. The Karroo shales and sandstones 

 contain a considerable quantity of iron pyrites in the form of 

 crystals and thin films colouring the rocks, and it is easy to see how 

 a sulphate could be derived from this salt ; where there is lime on 

 the surface the elaboration of a sulphate is very apparent, for the 

 lime becomes gypsum, leaving the iron in the form of red or yellow 

 ochre. Given, then, ferric sulphate and alluvial gold, it is easy to 

 follow the process of the building up of the gold octahedra and 

 nuggets generally. 



When it rains most of the water instantly flows away ; a little 

 remains and soaks in, absorbing the ferric sulphate and afterwards 

 dissolving a certain amount of gold. This then gets drawn into 

 crevices beneath the sandstones, lodges in hollows in the quartz- 

 reefs where the crystals do not occupy the whole width of the 

 fissure, in the cavernous portions of bones, and finally in the sand 

 on the bottom of the creeks. Here it has ample leisure to corporate 

 during the prolonged droughts, and the crystalline gold is the result. 

 I look to the more frequent application of evaporation to explain 

 the occurrence of gold crystals in the Karroo goldfields ; in Knysna, 

 where there is never a real drought, crystals have never been found. 

 Some of the gold nuggets are powdered with red gold, which 

 perhaps is the gold precipitated from solution by the reduction of 

 the ferric salt to the ferrous one, the action being too quick to allow 

 of the coherence of the particles. In the sand the gold has been 

 precipitated in the form of minute spicules. 



:E^ IE "V IIB -VT S- 



I. — The Museum Humfredianum, 1779. By C. Davies Sherborn. 



ALL interested in Conchology will be glad to know that a copy of 

 this work has at last been discovered in the Hancock Museum, 

 Newcastle-on-Tyne, and the mystery as to whether or not it 

 contained generic and specific names is now finally set at rest. The 

 Catalogue of the Humphrey Museum is of no scientific value 

 whatsoever. I am printing a full description of this rare tract in 

 the Annals and Magazine of Natural History, and as the Editor 



