1868.] Mineralogy, Mining, and Metallurgy. 101 
Jaws of mineral deposits. As an illustration of these advanced 
views, our author refers to his own researches which, although con- 
fessedly imperfect, tend to the remarkable conclusion that, in 
eruptive rocks, most minerals present themselves under similar 
conditions, accompanied by the same associated minerals, and in 
rocks of corresponding geological age. Hence he believes that 
certain minerals may serve to identify contemporaneous outbursts 
of eruptive rocks, in the same way that fossils serve to determine 
the age of sedimentary deposits. When, as often happens, the same 
mineral occurs in rocks of different age, he finds that in each 
situation it is marked by distinctive characters of its own, either 
physical or chemical. Thus, mica is distributed through rocks 
varying widely both in character and age; but while in granite it 
usually occurs as muscovite or potash-mica, in limestone and 
serpentine it appears as phlogopite or magnesia-mica, in zircon- 
syenite as astrophyllite or titaniferous mica, and in volcanic rocks 
as biotite. Generalizations of this kind are sufficient to show that 
the field of labour which Mineralogy affords is after all not so 
unattractive as many are inclined to think. As an earnest that 
the author himself is willing to bear a fair share of the work, he 
gives us a paper devoted chiefly to a notice of British Gold. 
Although public attention has from time to time been directed 
to the occurrence of gold in this country, and considerable excite- 
ment has been aroused by the recent workings in North Wales,* 
no analysis of British Gold has hitherto been recorded. Mr. Forbes 
has therefore visited the Welsh mines, and has analyzed specimens 
from the celebrated Clogau lode with the following results :— 
Ife II. 
Gold. ne ud : ; 90°16 89°83 
Silver . - 5 : . 9°26 9:24 
Tron and Copper . . : trace. trace. 
Quartz . . : ; 0°32 0-74 
Loss, : - - ° 0-26 0°19 
100-00 100°00 
The metal from this mine is therefore an alloy of gold and silver, 
closely agreeing with the formula—Au, Ag. After fully describing 
the character of the lode in which it occurs, Mr. Forbes discusses 
the probable age of the gold. He has already classified all known 
auriferous veins in two great groups—the older or granitic, which 
were formed at some time between the Silurian and the Carbon- 
iferous period, and the newer or dioritic, probably of Cretaceous 
age: it is to the former of these classes that the author is inclined 
to refer the gold-veins of North Wales. 
* See a paper in this Journal on “ British Gold with especial reference to the 
Gold Mines of Merionethshire, by Robert Hunt, F.R.S.,” vol. ii, p. 635. 
