672 The American Naturalist. Lid 
when treated in the same way at 300°, yielded little crystals of corun- 
dum with pyramidal terminations. Quartz crystals with rhombohedral 
terminations were produced by heating pulverized glass or amorphous 
silica to 300° under the same conditions. Microcline gave Zrzdymife 
plates. A mixture of metallic iron, iron oxide and amorphous silica 
produced litte black amorphous plates of z/menite and crystals of 
magnetite. The syntheses are of importance as indicating the possi- 
bility of the formation of contact minerals at a low temperature in 
the presence of traces of fluorine. Johnston P has subjected mus- 
covite and biotite to the action of pure water and to that of dilute 
carbonic acid for the length of one year. The muscovite undergoes 
no change in this time except slight hydration, in consequence of 
which it becomes a hydro-muscovite resembling margarodite in com- 
position. Biotite during this time becomes bleached under the in- 
fluence of the carbonic acid. It loses some of its magnesium and 
iron, assumes water, and passes like the muscovite into a hydromusco- 
vite. Anhydrous micas when they undergo hydration increase in bulk, 
a fact that may help to explain the cause of the rapid weathening of 
micaceous sandstones. Bruhns # has succeeded in obtaining genuine 
&lass inclusions in quartz by heating in a bath of molten granite speci- 
mens of phrase and pieces of quartz containing inclusions of fibrolite. 
The resulting glass is entirely surrounded by quartz, and is not merely 
a portion of the granitic substance filling cracks produced in the 
mineral by heating. The inclusions are arranged in straight and 
curved lines, and have all the properties of inclusions found in porphy- 
ritic quartzes. A brief comparison of the shapes of the etched figures 
in diopside and spodumene is made by Greim?. The depressions 
found on the co P faces of the first mineral are nearly triangular, with 
their apices inclined toward the positive hemi-pyramid. Mr. Lane 
describes a method of determining the values of the optical angles of 
minerals in thin sections of rocks without the use of converged light. 
18 Quart. Jour. Geol. Soc., May, 1889, p. 363. 
19 Neues. Jahrb. f. Min., etc., 1889, L., p. 268: 
20 Miner. Magazine, May, 1889, p. 252. 
21 Amer. Jour. Sci., Jan., 1890, p. 53. 
