Table Mountain, near Golden, Colorado. 455 
2. Thomsonite. 
This zeolite followed closely upon the chabazite, indeed its 
earliest aggregates are so intimately associated with that min- 
n 
other like the leaves of a closed fan, and the very compact 
_ pena ee of such agerogates are usually arranged in a 
more or less distinctly radiate manner. -Sometimes ‘spherical 
rine result ; in other cases, solution, by radiation from an axis; 
or less frequently, walls, the blades standing at right angles to 
the centra] plane. here crystals of yellow calcite were not 
covered by feGasite the thomsonite has never failed to coat 
them, the blades being approximately perpendicular to the 
crystal hen a large surface of shahabite has been 
completely coated by the more or less radiate aggregates of 
thomsonite, forming an undulating surface, the whole has a 
most delicate silken janine vcs that on a fractured surface 
of a spherical mass is more satin-like. e€ aggregates are 
white when pure; single Sages are transparent 
It is eae to obtain isolated leaves, and they are then so 
thin o affect polarized light but slightly ; still, it was sot 
nitely sy aimee that the direction of total extinction betwee 
crossed Nicols is parallel to the longitudinal axis. By examin- 
ation of the ends of the blades, under the microscope, with a 
power of ninety diameters, it could be seen that the vertical 
edges were formed in many cases by a plane at right angles to 
ond to the 
prism and eR io rennet of tho rapt’ as 
authorities. The termination seems to be the basal ae 
The average thickness of these Bindas' is shout Toward 
the close of the zeolitic formation, a second generation “of 
thomsonite was deposited. The blades are in this 
than those of the first, while the other dimensions remain 
about the sam 
In combination with the basis upon the crystals of this latter 
growth are apparent brachydomes, whose angles with the 
r 135 
The blades of the second generation, when upon 
those of the first, have the same crystallographic vigoeate 2's 
and serve at all as_prolongations of the latter. hrough the 
clear. 
The long blades of the second growth of thomsonite often 
form bundles resembling rough prismatic crystals, and these 
Am. Jour. Scr.—Tuirp Series, Vou. XXIII, No. 188.—June, 1888. 
31 3 ‘ 
