sith iii iarmnetineerieiei enna 
AMERICAN 
JOURNAL OF SCIENCE AND ARTS. 
[THIRD SERIES|] 
e 
Art. XXXVIIL—On some points in Lithology ; by James D. 
DANA. 
I. ON some or THE CHARACTERS EMPLOYED IN DISTINGUISHING 
DIFFERENT Kinps oF Rocks. 
LirHo.oey is a department of Geology, rocks being the ma- 
terial in and through which geological problems are presented 
for study. The true aim of the science of lithology is to describe 
the kinds of rocks mineralogically and chemically, and to note 
down their distinctions, in such a manner as shall best contri- 
bute to the objects of geology ; and these latter objects include, 
the nature or composition of the mass. 
The defining of rocks is attended with special difficulties on 
account of their mutual transitions. From granite down the 
are, with very few exceptions, mixtures of minerals, as muc 
is the mud of a mud bank. They graduate into one another 
by indefinite blendings, as the mud of one mud bank graduates 
into the mud of others around it. In fact a large part of the 
Am. Jour. Sot.—Turrp A capa Vou. XVI, No, 95.—Noyv., 1878, 
