174 General Notes. 
with an orthorhombic or a monoclinic symmetry. Those of the lat- 
ter kind are triangular in shape and are supposed to owe their 
abnormal symmetry to twinning. — Mr. Cross^ has noticed striations 
in the cubic faces oi galena from the Minnie Moore mine, Bellevue, 
Idaho, which he believes to be due to twinning lamellae produced by 
the slipping of alternate bands of the mineral along gHding planes, 
as a consequence of pressure. The twinning planes lie in the zone 
between oo O oo and oo O — New methods for the detection of tin. 
caesium, and rubidium under the microscope are suggested by Streng^ 
The detection of tin depends upon the fact that KCe and Sn Ce yield 
a double salt, which crystallizes in little tabular orthorhombic crystals, 
which upon the addition of nitric acid pass over into octahedroas 
modified by icositetrahedrons. Caesium and rubidium chlorides with 
stannous chloride in hyrochloride acid solutions give crystals of the 
saaie shape as those of potassium and stannous chlorides, but in 
the case of caessium these are brightly polarizing, while in the case 
of rubidium they are monoclinic. The author also calls attention to 
the fact that all hydrofluoric acid sold as pure, even when care- 
fully made from cryolite, contains silica and cannot be used for the 
detection of this substance in small quantities,— Calcium carbonate 
readily decomposes solutions of aluminium salts in the cold, with 
precipitation of gelatinous aluminium hydroxide, which, in the 
presence of coloring matters absorbs these and becomes stained, 
Under the same conditions dolomite produces no change in the 
solutions unless it remains in contact with them for a long time. 
A knowledge of these facts induces Lemberg' to propose a method 
of distinguishing between calcite and dolomite in thin sections of 
rocks. The solution which he proposes for use is made by dissolv- 
ing four parts of dry aluminium chloride in sixty parts of water 
and adding to it six parts of haematoxylin campechianum. 
BOTANY.* 
Two Big-rooted Plants of the Plains. — Now and then some 
of the plants of the plains present odd characteristics not ob- 
served in some of the eastern regions. Two species native of the 
open plains atan altitude of from 2,000 feet abovethesea to the base of 
the Rocky Mountains are remarkable for their enormous roots. One 
1 Proc. Col. Scient. Soc. 1887, p. 171. 
s Zeits. d. deuts. geol^Gesell. XL., 1888, p. 357. 
