458 Transactions. — Geotofjy. 



Thames Goldfields. The bed-rock in this locality is a soft, grey- coloured 

 felspathic tufa, with pyritous leaders, weathering white and decomposing 

 into a white clay. On page 40 of the report alluded to, an analysis of this 

 rock is given in No. 4. 



The form assumed by the gold is apparently that of botryogen, a bisul- 

 phate of ii'on, which has been formed by the decomposition of pyrites. The 

 measurement under the microscope of the angle of the oblique prisms is 

 nearly 120°, the typical form of botryogen is 117° 34' ; the latter angle is 

 used in constructing figs. 1 and 2 in the diagram. Two crystalline forms 

 are apparent, the one oblique prisms with the acute angles modified by the 

 positive orthodome faces of the form 1 oo 1 (see fig. 1), the other oblique 

 prisms with edges replaced by faces of the form lice and oolf. In fig. 3 

 is shown a portion of the specimen, enlarged. This occurrence of gold in 

 the form of casts of botryogen indicates that the deposition of gold in this 

 district has extended over a great period of time, involving these changes, 

 or of its redeposition in this form. In either case it is important evidence 

 towards the more complete knowledge of the occurrence of gold in this 

 district. 



The following particulars are attached for convenience of reference of 

 the crystallographic symbols : — 



a. 

 The ortho-pinacoid faces \ 

 ,, clino ,, ,, I of the oblique prism. 



,, basal ,, ,, J 



The positive orthodomes of right prism on oblique rhombic 



base. 

 The faces of the oblique rhombic prism, 1st order. 

 The chinodomes of oblique prism on rhombic base, 2nd order. 

 I am indebted to Dr. Purchas, the finder of the specimen, for the oppor- 

 tunity of making the above description. 



Tennant and 

 Mitchell. 



Naumann. 



Dar 



loo 00 



coPoo 



u 



ooloo 



OP 







0000 1 



OP 







lool 



Poo 



i 



lloo 



QOP 



i 



oolf 



fPoo 



§i 



