Mineralogy and Petrography. 931 
2.8459 : 1 : 2.9696. 8 = 77° 23’. Their composition corresponds 
to that of the same mineral from other localities——Very pure idri- 
alite (C,,H;,,O,), crystals from Idria in Krain, consist of small plates 
bounded on two sides by parallel planes. The extinction against 
one of these sides is 5° 33’, and the optical angle of the min- 
eral, measured in oil, is 2H = 101° 20’. The interference figure 
is that of a biaxial crystal, with the bisectrix perpendicular to the 
surfaces of the plate.—A great many observations have been made 
by Niedmann! upon barite in order to determine the relations 
between the elasticity and the other physical properties of the 
mineral. He finds the direction of greatest elasticity to be per- 
pendicular to the plane of easiest cleavage. The shape of the 
curve representing the relative values of the coefficients of elasti- 
city for any given plane, corresponds in all cases to the symme- 
try of the orthorhombic system. 
MIscELLANEOUS.—In a late paper Dr. Cohen? discusses critically 
the arguments in fayor of the chemical theory of deposition for 
gold in placers, and also those in favor of its mechanical origin. 
He gives the main facts bearing on the subject, and shows that 
some of those whlch have always been cited as favoring the chem- 
ical theory, are of doubtful existence. It has been stated, for 
Instance, that placer gold always contains less silver than vein gold 
from the same region, Analyses of specimens from Buttons Creek, 
in the South African gold-fields, however, prove that this is not 
always the case. Dr. Cohen’s results are :— 
Au Ag Cu Insol 
Vein gold 94.48 5.16 ged x 
Placergold {95:02 1.60 1 o7 
He thinks that whereas placer gold, for the most part, is derived 
by the breaking down of gold veins, there is occasionally in addi- 
tion a deposition of the metal from its solutions. This, however, 
he regards as playing but a subordinate rôle in the formation of 
placer gold.—A ‘meteoric stone,’ found in the San Emigdio 
mountains in California, is composed of chondri of olivine and 
anstatite, imbedded in a base composed of the same minerals in a 
. fragmental condition. Nickeliferous iron constitutes 6'21 per 
cent. of the stone, It occurs in lumps and masses, often surround- 
ing the chondri. An analysis of the metallic portion yielded 11.27 
per cent. of nickel. 
‘ Ib., xiii., 1887, p. 362. 
, Mitth. d. naturw. Ver. f. Neuvorpom. u. Rügen, 19 Jahrg., 1887. 
Merrill: Am. Jour. Sci., June, 1888, p. 490. 
