150 The American Naturalist. [February, 
litic. After discussing the nature of the schists and gneisses of this 
region the author concludes his paper with the statement that the 
term gneiss should be used in a geological sense only when the origin 
of the rock designated by it is unknown. He further surmises that 
many gneiss areas will, upon close study, be found to be underlain by 
granites and variousschists whose nature can be learned. 
Instruments.—Leiss® describes a number of new instruments for 
the use of petrographers. They are made under the direction of Fuess of 
Berlin. The first instrument is a microscope with nicols so arranged 
as to be capable of revolution independent of the stage. The stand is 
constructed for the attachment of the ordinary accessories. The three 
models at present on the market are known as VII, VIIa, VIII. The 
second instrument is a simplified form of Federow’s universal stage, 
admitting of the revolution of an object about two axes while under 
the microscopic objective. The third is the apparatus planned by 
Klein to aid in the examination of thin sections immersed in liquids. 
The other instruments described are a compensator-ocular, invented by 
J. Amann, a mica-wedge after the pattern proposed by Federow, à 
vertical illuminator for use with opaque objects, a very simple micro- 
scope camera made to fit over the ocular of an ordinary microscope 
and an achromatic condenser for use with the same. 
Diller describes an improved form of Smeeth’s separating tube for 
the separation of rock powders by means of heavy solutions. The new 
tube possesses several advantages over the Harada and the Brogger 
tubes. 
Petrographical Notes.—The reciprocal relations existing betweeD 
hornblende and augite in plutonic and volcanic rocks is explained by 
Becke’ as due to the fact that hornblende contains a small percentage 
of constitutional water in its molecule, and that it can therefore be 
formed only under certain conditions of pressure and temperature when 
water is present ina magma. When the conditions of pressure and 
temperature are not right augite forms in place of hornblende. A fig- 
ure showing curves exhibiting the possibilities of formation of the two 
minerals explains the author’s views. 
A massive holocrystalline rock composed of biotite, cyanite and cor- 
dierite, with apatite and rutile as accessories, was discovered by Me 
7 Neues Jahrb. f. Min. B.B. X, 1896, p. 412. 
8 Science, June 12. 1896, p. 
®Sitzb. deutsch. naturw.-med. Ver. f. Böhmen. Lotos, 1896, No. 5. 
