228 The American Geologist. April, iy04. 
by professor Shimek as Polygyra multilineata, Pyramidula 
alternata, and Succinea obliqua, may with equal certainty be re- 
ferred to the genera Planorbis and Limnea of the family Lim- 
nseidse, and Paludina integra of the family Paludinidse. The 
aeolian theory would accordingly suffer the loss of its best sup~ 
port. 
ON THE CLASSIFICATION OF SEDIMENTARY 
ROCKS 
By Amadecs W. Grabau, New York.* 
All true classifications are based on development or the 
genesis of species. A species, however, in the only correct 
sense of the word, is monophyletic ; i.e., individuals of the 
same species can not develop independently from two ancestors 
in the same or distinct regions unless these ancestors also be- 
long to the same species ; i.e., have a common ancestor. In the 
inorganic kingdom, species (in the biologic or only true sense) 
are not recognized, for minerals of the same type may arise in- 
dependently from distinct solutions, or rocks from separate 
magmas, and yet be, so far as we now know, absolutely indis- 
tinguishable. Again, rocks of different generations may be mdis- 
tinguishable ; as, for example, a siliceous sandstone derived 
from a granite after the removal of all minerals except the 
quartz, and one derived by the erosion of a sandstone and the 
re-deposition of the sand grains thus derived. In general 
terms, there is no continuity of development in rocks and min- 
erals, and no progressive differentiation, such as characterizes 
the organic world. 
Thus it will be seen, that with our present conception of 
rocks and minerals, a true genetic classification based on the 
inorganic materials is not to be hoped for, — nor would it be de- 
sirable, since it would compel us to distinguish between mater- 
ials otherwise exactly alike, but of independent origin. Xever- 
theless as far as practicable the principles underlying a genetic 
classification should be applied, and this can readily be done, if 
•A paper read before the New York Academy of Sciences November 16, 
1903. The author is under obligation to professors J. F. Kemp and W. O. 
Crosby, and doctors A. A. Julien, H. S. Washington and D. W. Johnson for 
helpful suggestions and criticisms. 
