PETROLOGICAL ABSTRACTS AND REVIEWS 279 
RINNE, F. “Beitrag zur optischen Kenntnis der kolloidalen 
Kieselsdure,”’ Newes Jahrb. Min., Geol., u. Pal., B.B. XXXIX 
(1914), 388-414, figs. 12. 
Describes apparatus and results obtained by the examination at 
different temperatures, between —600° and +1000°, and by light of 
different wave lengths, of various amorphous silicates, such as quartz- 
glass, hyalite, precious opal, moldavite, obsidian, and marekanite. It 
was found that these substances fell into two groups, water-free or 
water-poor, and water-rich. In the former the refractive indices pro- 
gressively increased with temperature changes, while in the latter they 
increased to the neighborhood of o° and then decreased. 
RINNE, F. “Die Kristallwinkelverinderung verwandter Stoffe 
beim Wechsel der Temperatur. I,’ Centralbl. Min., Geol., u. 
Pal., 1914, 705-18, figs. 9. 
With the apparatus described in the preceding paper, the author 
found that the angle of the rhombohedron (1011) in calcite, dolomite, 
siderite, and rhodochrosite increased with increasing temperature. 
Above o° the curve is a straight line, below o° it is slightly curved. The 
plagioclase feldspars show a decrease in the angle oo1-oro with increasing 
temperature. Albite shows the greatest change, anorthite the least. 
The curves are very flat at low temperatures and rapidly drop at high 
temperatures. 
ScHMIDT, EpuaRD. “Die Winkel der kristallographischen Achsen 
der Plagioklase,” Chemie der Erde, I (1915), 351-406, figs. 13, 
bibliography. - 
A study of the plagioclase feldspars, unusually valuable since the 
material was analyzed. There are new determinations of the cleavage 
angle (oo1):(o10), which show that this angle is a linear function of the 
An. content. The specific gravity determinations, with one exception, 
agree very well with the determinations made by Day and Allen on 
artificial feldspars. The value for the labradorite from Labrador is 
given as 2.689+0.003, and its composition as An. between 49 and 50; 
the artificial feldspar Ab,Any gives a value, according to Day and Allen, 
of 2.679. The material from Labrador, however, was zonal, and the 
An. percentage as computed from the silica, lime, alkalies, etc., varied 
between 44.8 and 55.8. It is possible, therefore, that the anorthite 
