C 331 3 
XXIII. Experiments on a Mineral Substance formerly supposed to 
be Zeolite ; with some Remarks on two Species of Uran-glimmer. 
By the Rev. William Gregor. Communicated by Charles 
Hatchett, Esq. F. R. S. 
Read July 4, 1805. 
This mineral is raised in a mine called Stenna Gwyn, in the 
parish of St. Stephen’s, in Branwell, in the county of Cornwall ; 
the principal production of which is the compound sulphuret of 
tin, copper, and iron. 
Description. 
Two species of this mineral are found, assuming a marked 
difference in external character. 
The first and most common one consists of an assemblage of 
minute crystals, which are attached to quartz crystals, in tufts, 
which diverge from the point of adherence, as from a centre. 
These tufts vary, as to the number of crystals, of which they 
are composed, and are light and delicate in the forms which 
they assume, or they are grouped together according to a 
variety of degrees of proximity and compactness. Sometimes 
they fill the whole cavity of a stone, with little or no interrup- 
tion ; in other specimens they are seen partially spreading over 
the sides and pointed pyramids of quartz crystals. 
In some cases these grouped tufts adhere very pertinaciously 
to the stone which bears them ; in others, they are easily se- 
parable, in comparatively large pieces, from the quartz, he 
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