ae DESCRIPTIONS OF MINERALS, 
especially at Vesuvius, where crystals occur from the size of 
a pin’s head to a diameter of an inch. Also found in the 
Leucite Hills, northwest of Point of Rocks, Wyoming Ter- 
ritory. 
The name leucite is from the Greek lewkos, white. 
FELDSPAR GROUP. 
I. RELATIONS OF THE SPECIES OF FELDSPAR. 
The species of the Feldspar Group are related— 
A. In crystallization: (1) the forms being all oblique ; 
(2) the angle of the fundamental rhombic prism J, in each, 
nearly 120°; (3) the other angles differing but little, al- 
though part of the species are monoclinic and part tri- 
clinic ; and (4) there being two directions of easy cleavage, 
one, the most perfect, parallel to the basal plane O, and the 
other parallel to the shorter diagonal section, with the in- 
tervening angle either 90° (as in the monoclinic species or- 
thoclase and hyalophane), or nearly 90° (as in the triclinic 
species). 
B. In composition: (1) the only element in the sesquiox- 
ide state being aluminum, and those in the protoxide state 
either calcium, barium, sodium, or potassium, or two or 
three of these bases together; (2) the ratio of 1 atom of R 
to 1 of R beimg constant ; (38) the amount of silica in the 
species increasing with the proportion of alkali, being that 
of a unisilicate in the pure lime-feldspar, anorthite, that 
of a tersilicate in the soda-feldspar, albite, or potash-feldspar, 
orthoclase, and so directly proportioned to the alkali, that 
the amount in any lime-and-soda feldspar may be deduced 
by taking the lime (or calcium) as existing in the state of a 
unisilicate, and the soda in that of a tersilicate, and adding 
the two together. 
Anorthite has the formula CaAl O, 81.. 
Albite oe ge Na, Al O,, Sig. 
The constitution of a species containing Ca and Na, in 
the ratio of 1 to 1 for the protoxide portion may be ob- 
