126 CONTRIBUTIONS TO MINERALOGY. [bull. 262. 
The mineral is easily decomposed by hydrochloric acid. Certain anal- 
ogies can be noted between heulandite and gyrolite, and from this and 
the properties of the mineral, as far as known, the writer believes 
gyrolite to be monoclinic. Heddle says in connection with some plane 
angles he measured on gyrolite from Scotland: "The above, as far as 
it goes, seems to show gyrolite to be oblique, and near heulandite in 
form." 
The formulae of the two minerals are also similar. 
Gyrolite 68i0 2 . 4CaO. 5H 2 0. 
Heulandite 6Si0 2 . A1 2 3 . CaO. 5H 2 0. 
6. APOPHYLLITE. 
With the gyrolite were found a number of specimens of clear, trans- 
parent, but very minute crystals of apophyllite. These are usually 
less than 1 mm. thick and show only the base, prism of the second 
order, and the unit pyramid of the first order. Some years ago a] 
specimen was obtained here on which the apophyllite crystals have 
been changed to quartz. As they proved to be infusible, a more 
extended investigation of them was made. 
The crystals, from 1 to 3 mm. in diameter, appear cubic, with the 
corners truncated by small faces. Measurements of the best crystals 
showed that they are tetragonal and that they agree in angles with 
those of apophyllite. The prism faces are vertically striated and faces 
of the other two forms are also rarety somewhat striated. 
By trial it was found that hydrochloric acid does not attack the 
crystals, so that the associated calcite was easily removed and pure 
material for analysis was obtained. The crystals are opaque, and no 
optical determinations could be made. 
The analysis gave the following figures, showing that the crystals 
are now, chemically, essentially silica with various impurities: 
Si0 2 90. 58 
A1 2 3 1. 58 
CaO 1. 87 
MgO 2. 20 
H 9 4. 32 
100. 55 
TEi/LtmrrE. 
Several small crystals of tellurite were referred to the writer by 
Doctor Hillebrand. Their locality is the W. P. H. mine, Cripple 
Creek, Colo. The crystals are transparent yellow and reach a length 
of about 2 mm. They are thin tabular parallel to the brachypinacoid. 
This form is normal to a bisectrix, and the axial plane is parallel 
to a (100). 
