I589.J Mineralogy and Petrography. 909 
angle of 72° 12', a sp. gr. of 5.5 7, and a cinnabar — red streak; 
the latter with 71° 22' as the rhombohedral angle, a specific gravity 
of 5.85, and a purplish red streak. (2), Pyrargyrite twins parallel to 
m, R, 00P2— iR, and oR, proustite parallel to JR, R, oR 
and — iR. (3), Both minerals are rhombohedral, and none of 
the typical forms exist at the same time in both plus and minus 
positions. (4), Pyrargyrite is 3Ag,S.Sb,S3, with a small per- 
centage of As, and a:c=ii ■..'j?>()2', and proustite is 3Ag2SASjS„ 
with occasionally a small amount of Sb, and «.•<:.= i : .8038. Many 
new forms are described in pyrarygrite and a few in proustite. 
Fine crystals of gold from the placers of the Senarka and the Kam- 
enka rivers, Gouvernement Orenburg, Russia, are described by Jere- , 
mejew^ in the Goruyi Journal iox 1887. Combinations of ooOco , O, 
and other more complicated forms are beautifully twinned parallel to an 
octahedral face. Pseudomorphs of limonite after pyrite, anatase, 
rutile, and many other interesting minerals associated with the gold, are 
also carefully described and figured. Molengraff^ has contributed 
to the study of quartz an interesting article on the rounded faces on 
many crystals. Since these are nearly always covered with concavities 
of the shape of the etched figures characteristic of quartz, the author 
regards them as having been produced by natural etching. The effect 
of alkaline carbonates, hydrofluoric acid, and potassium hydroxide 
upon crystals has been studied, resulting in the discovery that alkaline 
carbonates at 125° quickly produce etched figures in such a way that 
the etched faces are rapidly rounded. According to Valentin'" the 
abnormal crystallographic development of certain barite crystals from 
the Kronthal, in Alsace, is also due to natural etching. Concavities 
occurring on P<» , oP, and J P^ are identical with those produced by ar- 
tificial means. After discussing the analyses olatacamite that have been 
published within the past ninety years, together with new analyses pub- 
lished by himself, Darapsky " concludes that the substance called by this 
name is probably an isomorphous mixture of molecules varying within 
the limits CUCV3CUO.3H,, and CuCV4Cu0.60H,0, just as plagio- 
clase is an isomorphous mixture of the albite and anorthite molecules. 
Having obtained a few large, pure crystals of pseudobrookite from 
Havredal, Bamle, Norway, Cederstrom ^ has analyzed them, and found 
8 Ref. Zeits.fur Kryst., XV.. p. ^26. 
