J. D. Dana on the Characters distinguishing Kinds of Rocks. 439 
supposed to give to lithological system would be greater if “ feld- 
spar” were substituted, and with its present range of constitu- 
tion, the evil would be hardly less. 
7. Rocks differing mineralogically, and not chemically, like 
related hornblendic and augitic rocks (the minerals hornblende 
and augite being dimorphous), are rightly made distinct rocks, 
since the difference has depended, to a large extent, on wide- 
reaching geological operations or conditions, and is, therefore, ° 
of great geological signficance. 
Since quartz is the most widely distributed and therefore 
the least distinctive of the minerals of rocks, it may rightly be 
regarded as of subordinate importance in the distinguishing of 
rocks, and hence not only such names as dioryte and quartz-dio- 
ryle, trachyle and quartz-trachyle, etc., are acceptable, but also 
eing, 
constitution and formula, the rocks in which biotite is a chief 
Constituent cannot rightly be put in the same group with 
hornblende rocks; or those in which hornblende is a chief 
Constitution. Lithology is to receive hereafter its greatest 
advances through chemical analyses; for chemistry alone can 
clear away the doubts the microscope leaves, and so give that 
completeness to the Science of Rocks which geology requires 
for right and comprehensive conclusion 
Moreover the researches made in the laboratory to be of real 
Seological value should be, if possible, supplemented by inves- 
tigations in the field as to transitions among the rocks, and as 
to other kinds of relations. This field work has often been 
well done, but not so by all lithological investigators. — 
_ The principles presented lead to. the following subdivisions 
in an arrangement of crystalline rocks, exclusive of the Calca- 
reous and Quartzose kinds. Since leucite is a potash-alumina 
Silicate, like orthoclase and microcline (it affording twenty per 
cent or more of potash), it is here referred to the same grou 
with the potash feldspars; and nephelite, sodalite and the 
Saussurites being eminently soda-bearing species, they are 
