SCHALLKR.] 
MINERALOGICAL NOTES. 
125 
scopic fissure of the rock. A multitude of these veins are present, 
lying in nearly parallel position and varying greatly in size. There is 
nothing to suggest that the gyrolite may have been derived from 
apophvllite. It may have been derived entirely from the igneous 
rock or only in part. The mineral was first deposited at the walls of 
the vein and from thence grew outward into the open fissure from 
both sides, also probably replacing the rock wall. The thin section, 
as well as the hand specimen, shows a decided plumose structure of 
the mineral. These plumose aggregates show, under crossed nicols, 
the black extinction cross. The interference colors are bright and 
vivid. Heddle a sa}^s of these: u The sections of orbicular gyrolite 
are among the most splendid of polariscopic objects, resembling a cir- 
cularly opened fan, of most brilliant and abruptly contrasted colors." 
The properties of the mineral are not very well known. Heddle 6 
mentions some crystals resembling heulandite in form. The mineral 
is described as unaxial, though as a matter of fact it is biaxial, but 
like biotite the axial angle becomes 0° at times. Cleavage plates 
show the emergence of a bisectrix. The (approximate) measurement 
of 2 ENa gave values (on the Fort Point material) of from 0° to 25°. 
The sign is always negative. Heddle obtained 2 E = C — 3°. When 
there was a straight edge suggesting a possible crystal direction, the 
extinction was not parallel. 
Dana c gives as the formula of the mineral 3H 2 0, 2CaO, 3Si0 2 , 
while Clarke d gives 5H 2 0, -iCaO, 6Si0 2 , which is the formula deduced 
by the writer from the following analysis of the San Francisco 
mineral: 
Si0 2 . 
A1,0 3 
CaO . 
Na 2 
H 2 0. 
53.47 i 
.22 !. 
32.00 ] 
1.25 |/ 
13.21 
100. 15 
6.02 
4.02 
4. W 
The specific gravity is 2.39. Traces of iron, magnesium, and 
potassium are present, while strontium, chlorine, and fluorine are 
absent. 
" Heddle, M. F., On the optic properties of gyrolite: Min. Mag., vol. 9, 1890-1891, p. 391. 
''Heddle, M. F., On the crystalline form of gyrolite: Min. Mag., vol. 8, 1888 1889, p. 272. 
cSystem of Mineralogy, 6th edition, p. 566, 1892. 
dClarke, F. W., The constitution of the silicates: Bull, 1'. S, Geol, Survey No, 125, 1895, p. 81, 
