113 
TAB. CCLVII. 
SILEX  Petuntse. 
Feldspar. 
Tue specimens here figured, from Cornwall, are curious, 
this formation not having till lately been known as British. 
Such specimens are very rare at present; the crystals not 
being imbedded, as is most usual ; besides which, they much 
resemble the Feldspar commonly termed Adularia in ge- 
neral habit, although not so transparent as those commonly 
brought from the place which gave rise to that name. They 
are, however, whiter and more delicate than usual in those 
of British origin. | 
The upper specimen has many small crystals upon it, 
among Quartz and Chlorite, or Green Talc, which are 
scattered about the gangue, and occasionally crowded or 
grouped; but more commonly like the right hand middle 
jgure. 
The left hand middle geometrical figure shows the posi- 
tion of the primitive rhomb within the crystal here figured, 
two prismatic faces of which correspond with it, and the 
little faces of the corners correspond with the terminal 
faces of the primitive. The broader striated face forms an 
angle of about 90° upon the vertical edge of the prism, 
and the faces of the prism an angle of about 1281° upon 
each other. het 
The lower figure represents a valuable specimen from 
Huel Rose mine in the parish of Gwennap, Cornwall, which 
was lent me by my kind friend John Williams, Esq., who 
is fortunate in possessing many peculiarly curious Cornish 
minerals, 
VOL. Ill. I 
