438 J. D. Dana on some points in Lithology. 
CONCLUSIONS. 
The principal points with regard to rocks which have been 
brought out in this paper, are the following. 
1. The necessities of the science of Geology constitute the 
most prominent motive for distinguishing Ainds of rocks; 
and they should determine to a large extent upon what charac- 
ters distinctions should be based. 
2. In determining the rocks to be grouped as one in kind 
under a common name, near identity in the chemical and min- 
eral composition of the chief constituents is the main point to 
be considered ; not near identity in their crystalline forms, for 
isomorphism presupposes diversity of composition. 
8. Distinction of kind should be based on difference in chem- 
ical and mineral constitution as regards the chief constituents. 
When such difference exists, rocks are different in ind, an 
need, for the purposes of geology, distinct names. If it does 
not exist, the distinction is only that of variety; unless (as in 
the case of trachyte and felsyte), the very wide extension of the 
rock under persistent characters makes a distinction of name 
important to geology. 
4. It follows from the preceding, that differences in texture: 
as coarse, or fine, or aphanitic ; porphyritic, or non-porphyritic ; 
stoney throughout, or having unindividualized portions among 
6. Since “plagioclase” is not the name of a mineral species, 
——several minerals, of widely different compositions penis 
microcline, a large part of potash feldspar, which had been sup- 
posed to be orthoclase, it has become almost synonymous with 
the term feldspar. The “ simplicity” its adoption has been 
