6^ On a native Arsenlate of Lead. 



\n the county of Cornwall, so justly celebrated as well for 

 the variety as for the richness of its mineral productions, 

 will not be deemed superfluous. 



This mineral was raised in the mine called Huel-Unity^ 

 a very rich copper mine, in the parish of Gwennap. Ac- 

 cording to the information with which 1 have been favoured 

 by Mr. William Davy, c. very intelligent and experienced 

 miner in that district, it was found in a lode south of Muel- 

 Unity principal lode, at the depth of fifty fathoms below 

 the surface, which lode underlay about two feet in the 

 fathom south : at the depth above mentioned, this lode fell 

 in or formed a junction with another small lode or vein to 

 the south, and when the junction look place this lead ore 

 was found. The veins of it are, in general, from six to 

 ten inches wide, and they diverge on going west. Some 

 particles of this lead ore have been found in tlie southern 

 part, after tha separation of the lodes ; but the northern 

 lode does not contain any until the junction takes place. 

 This ore is intermixed with some native copper, very rich 

 gray copper, and black copper ore, and some is mixed with 

 quartz. The walls of both veins are killas. 



II. Description, 



Tliis mineral is regularly crystallized. The form of its 

 most perfect crystals is an hexahedral prism , they are of 

 different sizes, from one-tenth of an inch in diameter to 

 the size of a hair. The longest which I have seen do not 

 exceed three-tenths of an inch in length : these terminate 

 in a plane, at right angles, with the axis of the prism ; but 

 the crystals of a smaller size are frequently drawn out into 

 a very taper acumination, which appears to be a six-sided 

 pyramid. A number of smaller crystals are often closely 

 packed together in bundles, which are bent in different di- 

 rections, and terminate in a point. The larger crystals 

 cither stand alone, or adhere, on their lateral planes, to the 

 gangue, or are confusedly matted together in a mass. 



Some of them are hollow, as if an internal nucleus had 

 been destroyed ; and sometimes this internal nucleus over- 

 tops the external laminae. The gangue is a white quartz, 

 which frequently exhibits on its surface the appearance of 

 a partial decomposition. 



The red octahedral copper ore, and the copper into which 

 that ore passes, are often intermingled with the crystals of 

 this lead ore and imbedded in them. 



The colour of these crystals consists of a variety of tints 

 of yellow. Some are of a beautiful wine yellow resembling 



the 



