1 889-] Mineralogy and Petrography. 525 
interfacial angles he concludes that the values of these are the 
means of the values of those of the two simple sulphates, calcu- 
lated in the proportions of their molecular combinations. The 
author proposes to study other mixed salts in the same way. 
McMahon' makes use of a thin quartz wedge for the determi- 
nation of the strength of the double refraction of minerals in 
their rock sections. The quartz wedge is inserted between 
the crossed nicols of a microscope at an angle of 45° to their 
planes of polarization, and the point is noted at which there is 
no double refraction apparent when the object under investi- 
gation is placed on the microscope stage. This point of no 
double refraction is indicated by a dark line crossing the field. 
Its position varies with the strength of the double refraction 
of the mineral, so that by comparing its distance from the end 
of the quartz wedge with the distance observed in the case of 
minerals of known strength a ready means is afforded for a 
rapid determination of its double refraction. 
Heririg' mentions the existence of a grotto in the Wasch- 
gang Mine at Dollach in Corinthia, whose walls are covered 
with well formed ice crystals, some measuring as much as 200 
mm. in diameter. — Fulgurite glass from lightning tubes in a 
glaucophane epidote schist, in which occur yellow garnets, 
sphene, and occasionally diallage, is described by Rutley 
from the top of Monte Viso. The interesting fact in connec- 
tion with this fulgurite is the existence in the tubes of a vesi- 
cular glass in which gas bubbles, and globulites and microlites 
are scattered. 
In an article entitled the " Physics of Metamorphism," 
Harker* calls attention to the influence of pressure in effecting 
changes in the character of rock masses, and divides meta- 
morphism into hydrothermal dynamo, and plutonic metamor- 
phism, the meaning of each of which terms he explains in 
some detail In a red copper slag from the Canton Copper 
Works, Baltimore, Messrs. Jarman and McCaleb' have discov- 
ered cuprite in little octahedral crystals. 
MiNERALOGlCAL NEWS.— Much additional" knowledge in 
regardto the sulphates occurring near Copiapo, Chili, has 
