
824 The American Naturalist. [September, 
MINERALOGY AND PETROGRAPHY.! 
Petrographical News.—lIn an argumentative article on the 
individuality of rocks Lang? proposes to regard asa rock individual 
the product of a continuous (uninterrupted) rock-forming process. 
Accordingly he would class interbedded clays, sandstones, and con- 
glomerates deposited from the same body of water by a gradual lessening 
of its velocity, as a single rock, With clayey, sandy, and conglomeratic 
facies. In the eruptive group, that is a rock individual which has been 
forced from the depths of the earth by a single earth-throe. The 
beginning and end of a rock-forming process thus marks the limitations 
of arock individual, even though a second period of similar processes 
should subsequently give rise to a rock of the same nature as that pre- 
viously formed. The ideas discussed in the article are of great interest, 
but the practical advantages to be gained by regarding rocks from the 
point of view of its author are not immediately perceptible. 
_ Another article of some theoretical interest is Justus Roth’s criticism 
of Rosenbusch’s* recent paper on the chemical nature of eruptive 
rocks. After giving a short historical review of the attempt to explain 
the variation in eruptive rocks upon a chemical basis, the author * pro- 
ceeds to examine critically the views set forth by Rosenbusch. In the 
first place, he states that ten of the sixty-three rocks whose analyses are - 
quoted by this writer are much altered from their original condition ; 
others are but local in distribution, while of others, again, the analyses 
are incomplete. He further continues by showing that the fundamental 
magmas by whose mixture the various types of rocks are regarded as 
made up, far from giving rise to definite varieties when mingled in 
definite proportions, may themselves consist of different combinations 
of different minerals. He then calls attention to what appear to be 
weak points in Rosenbusch’s calculations, and concludes with the state- 
ment that there is not yet sufficient knowledge concerning the chemical 
character of eruptive to warrant the construction of a theory concerning ` 
their Loewinson-Lessing® has attacked the chemistry of 
the eruptive rocks in a little diffetent way from Rosenbusch. He com- 
pares the relations between the bases and the acid in a rock, and calls . 
1 Edited by Dr. W. S. Bayley, Colby University, Waterville, Maine. 
2 Miner u. Petrog. Mitth., X1., p. 467. 
3 AMERICAN NATURALIST, 1890, p. 1071. 
4 Zeits. d. deuts. geol. Ges., 1891, p. 1. 
5 Bull. Soc. Belge de Géol., etc., IV., 1890, p. I. 






