458 Transactions.— Geology. 
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 iron, 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 11 (see fig. 1), the other _ 
prisms with edges replaced by faces of the form 1lm and w13. 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 :— 
Soe a Naumann. Dana. 
low a ii The ortho-pinacoid faces 
ala OP 0. ,, clino i », of the oblique prism. 
aol OP O_,, basal - ‘e ; 
lol Po i The positive orthodomes of right prism on oblique rhombic 
base. 
llo oP i The faces of the oblique rhombic prism, 1st order. 
wlZ Pa 3i The chinodomes of oblique prism on rhombic base, 2nd order- 
I am indebted to Dr. Purchas, the finder of the specimen, for the opp 
tunity of making the above description. 
