Classification of Rocks. — Spurr. 219 
expressed without encumbering the nomenclature with useless 
and misleading names. For example, if a dike rock be de- 
termined, in a thin section, to belong to a definite mineralogic 
and structural type such as basalt, we may refer to the Paleo- 
zoic basalt dikes on the Yukon ris^er, in Alaska, without en- 
cumbering nomenclature with a special name for dike basalt, 
with another special name for dike basalt of Paleozoic age, and 
with a third special and distinct name for a dike basalt of 
Paleozoic age as exemplified in the Yukon district. Such mul- 
tiplication of nomenclature is the reverse of useful. 
The writer is strongly in favor of a method of printing 
compound rock names which shall indicate clearly the value 
of each part of the designation. Thus, group names should be 
printed in capitals ; the names of the ferromagnesian minerals, 
determining the species and varieties, in ordinary type ; and the 
structural designations, determining" the types, in italics; thus, 
augite STENYTE apLyte^ diallage olivixe-basalt, hornblende 
DiORYTE porphyry. 
II The General Principles of Rock Classification. 
Although the writer's classification, as above noted, was 
independently evolved, yet it is both just and justifying to 
sketch out how far the principles therein contained are bor- 
rowed, or are identical with those of other writers. Many of 
these principles will be found asserted by eminent authorities 
and denied by others. 
Teall* enumerates the possible principles of rock classifi- 
cation as (i) chemical composition, (2) mineral composition, 
(3) texture, (4) mode of occurrence, (5) origin, (6) geologic 
age (distribution in time), (7) locality (distribution in space). 
It will undoubtedly be agreed, upon all sides in general, 
that all of these principles should be considered in obtaining 
a definite idea of the e.xact nature and relations of any rock. 
Rut as it has been proven impossible, after some attempts, to 
find out a uniform scheme by which all these characteristics 
shall agree, that is, that a given mineral combination shall 
have a constant chemical composition, te.xture, mode of oc- 
currence, origin, age, and locality, or even that any two of 
*British Petrography, Birmingham, 1886 p. 6 
