1887] Mineralogy and Petrography. 275 
cussion of the several stages in the alteration of pyroxene ande- 
sites, as illustrated by the specimens examined. In the case of 
the mica-dacite glasses, alteration begins along the perlitic cracks, 
when it produces globiform masses, and then gradually extends 
outward until the entire body of the rock becomes white and 
opaque and appears to be isotropic. The author thinks that this 
alteration product may be a hydrated acid glass, corresponding to 
the palagonite of basic rocks. In a recent article in which are 
In a letter to the Neues Jahrbuch 
Sir Mineralogie, Siemiradski? describes three anorthite rocks from 
the island of St. Thomas, one of the Antilles. One is a corsite 
with a ground-mass saturated with secondary opal, which has 
been produced by the decomposition of the other constituents. 
The other two are dyke rocks cutting the corsite. They can be 
best characterized as altered anorthite andesites. 
Mineralogical News.—The optical investigations of Lange- 
mann 3 on harmotome, phillipsite, and stilbite seem to indicate that 
the plane P% produce eightlings, with a quadratic symmetry; 
and, finally (3), three eightlings with oo P as their twinning plane 
yield twenty-fourlings with a regular symmetry.——By observing 
mineral is like that of cuprite and salammoniac, in the gyroidal 
hemihedral division of the regular system. The bromide and 
the iodide of potassium crystallize similarly. n a late num- 
ber of the fourth Beilage Band of the Neues Fahrbuch fir 
Mineralogie H. Schedtler5 has an elaborate paper on the thermo- 
electrical relations of tourmaline. The paper opens with an his- 
torical introduction to the subject, in which the results of many 
earlier investigations are given. Then follow descriptions of 
the methods in use for the detection of electricity in minerals, 
and some general considerations, after which the author de- 
scribes his own results based upon the examination of sixty- 
1 Min, u. Petrogr., Mitth. viii., 1886, p. 1. 
2 Neues Jahrb. f. Min., etc., 1886, ii. p. 175. 
3 Ib., p- 83. a ID., Vol, i Ds 234; 
s Ib., Beil. Band iv., 1886, p. 519. 
VOL, XXI.—NO. 3» 19 
