29 



It is intended eventually to arrange here a series of specimens Case 50, 



sliovvino- gold in association with various minerals. Minerals 



associated 

 9167, gold on cff/cjY^. witkgold. 



9169, gold on talc. 



9091, gold on galena. 



All from the Gilmandyke Mine, Eockley. 



Prom here onwards the exhibits consist of ores that have to Gold ores. 

 be won by regular mining operations from the lodes, or other 

 deposits, in which they occur, and which have to be crushed and 

 submitted to a more or less complex treatment before the gold 

 can be extracted. In the great majority of cases, the ores con- 

 sist essentially of quartz that has been deposited in fissures or 

 other spaces in the rocks of the earth's crust. In all cases the 

 agent that has brought about the deposition is toater in some 

 form or other, not fire, as is so commonly supposed. Fissures 

 occupied by quartz frequently extend to great, even to unknown, 

 depths ; other deposits are more local in character. Various 

 terms are used in describing the nature of lode-quartz. It may 

 be glassij, vitreous, or milJcg in appearance ; 'white, blue, or red in 

 colour. If stained yellow or brown by the presence of small 

 quantities of iron oxide, it is ironstained ; when more iron is pre- 

 sent, it \& ferruginous. It may be cellular, honeycomhed, cavernous, 

 saccli'aroidal, or flinty. 



In the upper portions of a mine the quartz is generally iron- 

 stained or ferruginous, cellular or cavernous ; moreover, the gold 

 is free, and therefore easily saved. At lower levels this porous 

 character, and the iron oxide, disappear; the stone becomes 

 compact, and is usually accompanied by iron pyrites {pyritous 

 quartz) or other metallic sulphides. This change in the upper 

 levels is due to the action of water and air. In case 113, a set 

 of specimens has been arranged to explain these terms. 



Grold -bearing quartz veins are almost entirely confined to 

 the oldest rocks of the earth's crust. Cases are known in 



