J. J. Harris Teall—On Hypersthene Andesite. 345. 
macroscopically of glassy felspars (Mikrolin) imbedded in a ground- 
mass which has a resinous or semi-resinous lustre. Microscopically 
they consist of two generations of felspar, pyroxene mostly rhombic, 
magnetite, and a glassy base with various devitrification products. 
The principal microscopic differences lie in the minute constituents 
of the ground. Thus the ground-mass of the Cheviot rock abounds in 
very small hexagonal plates of a mineral, presumably hematite, whilst 
that of the Steinerne Mann rock contains small well-formed octa- 
hedra of magnetite and minute crystalline grains of pyroxene in a 
globulitic base. In both cases the ultimate base is a true isotropic 
glass. 
The following are bulk-analyses of these two rocks which have 
been previously published in this MaGazine. 
I. ties 
SiO, 63; Ol eye oree vin 6.90 
Al, O03 14-9 ek TOA 
Fe.0,; 4:7 6°50 
MnO o — trace 
CaO bee 4°8 Serius 7°82 
MgO DBO R RATE, 3°76 
NGS OW ye pecs AOR ets 3°80 
K,0 nahh ewe 1°9 nae 1:98 
Loss Aa a 4-0 eee 2°76 
100°1 160°:938 
I. Cheviot rock. Sp. Gr. 2°54. Analysis by T. Waller. 
II. Steinerne Mannjrock. Sp. Gr. 2°69. Analysis by myself. 
I now propose to give an account of some further researches into 
the characters of these rocks, which were carried out by myself in 
the Laboratory of the Royal School of Mines under the guidance of 
Dr. Hodgkinson. 
For the purpose of isolating the porphyritic felspars and the 
pyroxenes, the rocks were broken up in an iron mortar and the 
fragments passed through a series of sieves. The grains finally 
selected for treatment were those which had passed through a sieve 
having 50 meshes to the inch, and had been retained by one having 
100. The felspar was first obtained by taking up all the iron- 
bearing grains by means of a powerful electro-magnet, which was 
excited by five Grone’s cells; and the pyroxene was then isolated 
from the powder thus taken up by means of hydrofluoric acid in 
the manner recommended by Messrs. Levy and Fouqué.' 
Fichtelgebirge. It is defined by Rosenbusch (Massige Gesteine, p. 346) as a horn- 
blende-bearing diabase. As the Steinerne Mann rock does not appear to contain 
hornblende, the term seems inappropriate. 
1 If I were repeating this isolation, I should use the solution of bi-iodide of 
mercury and potassium. The following directions for preparing this solution may 
be of interest to readers of the Gronocgican Macazine. Mix together in a 
mortar carefully weighed portions of the two salts, bi-iodide of mercury and iodide 
of potassium in the proportion of 124 grammes of the former to 100 of the latter. 
Add the mixed salts gradually to a small quantity of water (500 grammes of the 
mixed salts will dissolve in 80 c.c. of water) in the cold. Then place the vessel 
containing the solution on a water bath.and evaporate till a film begins to form on 
the surface of the solution. Allow to cool, and filter. The most convenient way of 
