224 The Atnerican Geologist. October, 1900 
it is not strictly perpendicular to the composition plane 
^1^(021) of the Baveno twinning. The latter plane makes an an- 
gle of 48° with ^*'(oio) and cuts diagonally across the square 
crystal, cutting the other twinnings wherever it meets them. 
The extinction is not symmetrical with respect to the plane 
cy2{02i), being at y'" on one side and 22° on the other. (See 
Plate II, Fig. i). 
The hardness of the mineral is 6 to 6.5; its specific gravity 
varies somewhat; the pure glass-clear fragments show only 
a slight variation, between 2.696 to 2.709, but oftenest very 
near 2.701. Large grains often seem lighter than 2.696; even 
as light as 2.690, but in preparation for chemical analysis not 
a single one of the small grains floated in liquid of the gravity 
2,696. It fuses quietly at about 4, or 4^/2, to a glass which is 
colorless and transparent. The flame coloration is the clear 
red of calcium, but is not intense. With the spectroscope the 
sodium line is also very distinct. The color of the mineral 
has already been given, since the color of the mineral and the 
color of the rock are practically identical. Before the temper- 
ature of fusion is reached the cleavages open notably, which 
has the effect of whitening the mineral, and also makes it easy 
to crumble it afterward in the fingers. It will be recalled that 
in its pure condition it is light gray, sometimes greenish, in 
mass; and when weathered it becomes dull white, varying to 
greenish, or less commonly to reddish from deposited hema- 
tite. The beautiful iridescence common in this feldspar is ab- 
sent; cleavage surfaces are often bright and glassy. The 
streak is colorless. When quite pure it is transparent even in 
mass. In thin section it is perfectly transparent and color- 
less 
This triclinic feldspar never occurs in true automorphic 
crystal form in the rock under discussion; it is always wholly 
crystalline and massive ; with coarse granitic texture, a con- 
choidal fracture is easily obtained, but several common cleav- 
ages exist. The cleavage parallel to /(ooi) is much more 
highly developed than the others, and usually gives bright, 
lustrous surfaces. It is distinctly better than the cleavage par- 
allel to^''^(oio), which is, nevertheless, very well developed for 
this mineral. The latter cleavage produces faces which are 
usually much smaller than those parallel to /(ooi), and are dis- 
