1890.] Mineralogy and Petrography. 175 
capacity of analogous particles to form similar crystals. Similar or 
analogous particles are those built on the same plan, though different 
in composition. Starting with these general ideas the author discusses 
the character of the particles forming the silicates, and concludes by 
applying his deductions to the explanation of the formulas of the most 
important silicates. The examination !8 of senarmontite crystals in 
thin sections parallel to the cubic, octahedral and dodecahedral faces, 
shows that the apparently simple crystals are combinations of six 
orthorhombic crystals, and that the optical anomalies so frequently ob- 
served in the mineral are due to this intergrowth, or to twinning. 
Unlike the double refraction of some other apparently regular minerals, 
the anomalous action of senarmontite is not in the least affected in a 
temperature as high as Miigge has reinvestigated the sub- 
ject of pressure twinning in sphene, and finds the twinning plane to 
be in the zone between oo P and —P (DesCloizeaux’s position), oo 
not to coincide exactly with —2P, as determined by Williams. 
ascribes the striations frequently observed in the sphene of arise 
rocks to pressure, but is not able to produce them by artificial means. 
As a result of measurements made in crystals of zinc obtained by 
slow distillation of the metal in a vacuum, Williams and Burton*! have 
calculated the axial ratio to be a :c¢c= r: 1.3564. The crystals are 
hexagonal, with a probable rhombohedral symmetry, and isomorphous 
with arsenic, antimony, bismuth, and tellurium. In the pyroxene 
from Pinzgauer, Cathrein ™ has discovered the forms Pa, Pz, +P}, 
all of which are new to the species. In amethyst from the Zillerthal 
he has found the new planes 




wpn 1; piP, and $P? ] 
4 4 4 

while the forms most common to the mineral are absent. The new 
plane 4P is recorded by Césaro* as occuring on topaz from Sax- 
ony. 
Miscellaneous.—Retgers ™ has made a careful examination of the 
heavy solutions used for separating rock constituents with the endeavor 
17 Zeitschrift. fur Min., XVII., p. 25. 
18 Prendel, Min. u. Petrog. Mitth. X1., p. 7. 
19 Neues Jahrb. f. Min., etc., 1889, II.. p. 89. 
21 Amer. Chem. Jour., XI., p. 219. 
22 Zeits. f. Kryst., XVII., 1889, p. 19. 
23 Bull. Soc. Franc. de Min., XII., p. 419. 
24 Neues Jahrb. f. Min., 1889, II., p. 185. 
