British Mineralogy. 343 



parts where it traversed Lingula-beds unaccompanied by such 

 diabases. 



If we could suppose the workings on the lode in such a mine 

 carried down through the entire thickness of these Cambrian 

 grits into softer and more congenial strata, it might even be an- 

 ticipated that the lode might again be found quite as produc- 

 tive as before, irrespective of actual depth from the surface. 



From the consideration of the general characters of these 

 auriferous lodes and of the mineral association*, the author as- 

 sumes the probability of these lodes pertaining to what he has 

 termed the older or granitic intrusion of gold, which he supposes 

 to have taken place some time between the Silurian and the Car- 

 boniferous period f; audit is also believed that these lodes have 

 been injected from below, and not formed either by segregation 

 or infiltration. 



Stream-Gold from the River Mawddach. 



Through the kind offices of Mr. T. A. Read win a specimen of 

 the gold-dust washed out from the bed of the River Mawddach, 

 near Gwynfynydd, some eight miles above Dolgelly, was obtained 

 and submitted to examination. 



The portion received contained the native gold in the form of 

 small flattened elongated spangles, in size from about a pin's 

 head down. to almost dust, and of a rich yellow colour; it was 

 accompanied by abundance of fine black sand, supposed to be 

 magnetic oxide of iron from its being strongly attracted by the 

 magnet, but which on analysis turned out to be titanoferrite J ; 

 it also contained small particles of quartz, slate rock, mica, and 

 some small cubes and fragments of iron pyrites and galena. 



The specific gravity of the gold, carefully separated from any 

 other admixture, was found to be 15*79 at 60° F. ', quantity em- 

 ployed 22*75 grs. 



* Especially the presence of tellurium and bismuth compounds, which 

 are usually found in the gold veins of the old or granitic class. 



t The author (British Association Report for 1865, Sects, p. 52, and Geo- 

 logical Magazine, vol. iii. p. 385) classified the known auriferous veins as 

 respectively pertaining to two distinct epochs: — (1) the older or granitic, 

 having made its appearance between the Silurian and the Carboniferous 

 period ; (2) the newer or dioritic, about the Cretaceous period. In the 

 case of the Clogau lode there is no visible granite known in the district ; 

 but nevertheless it is supposed to belong to the same system of metallic im- 

 pregnation. 



% In the qualitative or quantitative examination of this mineral the fol- 

 lowing process is employed : — A portion in finest powder is fused in a pla- 

 tinum spoon with about eight times its weight of bisulphate of potash until 

 all the iroa is seen to have been taken up. After being perfectly cold, the 

 result is dissolved in a considerable amount of cold water ; to the clear so- 

 lution a few drops of nitric acid are added and the whole boiled for some 

 time, when the titanic acid, if present, will be completely precipitated* 



