the Arseniates of Copper, and of Iron. 171 
discovered in the county of Cornwall ; it was first found either 
in Carrarach mine, in the parish of Gwennap, or in Tincroft 
mine, in the parish of Allogan. Its matrix, like that of almost 
all the copper ores of this country, was siliceous, and con- 
sisted of a decomposed granite, of which the greatest part of 
the feldspar had passed into the state known by the name of 
Kaolin. It was accompanied with grey vitreous copper ore, fre- 
quently in considerable masses ; also with much black oxide of 
copper ; and with various oxides of iron. 
The arseniate here spoken of, which had never been found 
in large quantity, had ceased to exist in the above-mentioned 
mines, when Huel Gorland mine, lately wrought, began to 
enrich mineralogy with this uncommon substance. The ma- 
trix of this is likewise siliceous ; sometimes crystalline ; and 
sometimes in an amorphous mass. Here and there w r e find 
mixed with it, in greater or less profusion, all the known oxides 
of copper ; many of the argillaceous oxides of iron ; also grey 
vitreous copper ore ; arsenical pyrites ; and the rich deep- 
coloured yellow copper ore. This last is often found differing 
from its usual appearance, in a manner which I believe has not 
hitherto been taken notice of. I think it should make a distinct 
variety among the deep yellow copper ores, under the name of 
yellow hematitic copper ore. 
When the combination of copper with iron and sulphur is 
rich in metal, (for when it is poor, it is only a martial pyrites 
mixed with a little copper,) its appropriate colour, when a piece 
is fresh broken, is a deep yellow ; and this yellow colour is more 
deep, in proportion as the quantity of copper is more abundant. 
In its richest state, it assumes a more or less greenish tint. The 
surface of a fresh fracture is very brilliant, and appears rather 
Z 2 
