494 G. W. Tyrrell—Petrographic Nomenclatwre. 
Some Points in Petrographic Nomenclature. 
By G. W. Tyrrety, ARC Se; EGS. Wah...) Wecturersam 
Geology, University of Glasgow. 
R. ARTHUR HOLMES has laid all petrographers under a debt 
of gratitude for his Nomenclature of Petrology, which worthily 
succeeds and replaces Lowinson-Lessing’s Leaxique Petrographique. 
It represents an enormous amount of patient labour, and will 
provide an indispensable instrument of petrographic research. Its 
perusal has led the writer to consider several points in modern 
petrographic nomenclature which seem to need further discussion. 
There is no need to labour the extreme complexity, unevenness, 
and confusion of present-day petrological nomenclature. Dr. Holmes 
illustrates it in his Introduction by compiling a list of no fewer 
than twenty-two different principles and characters upon which 
names have been based. This state of affairs is attributed by him 
to two causes: the natural tendency for the meaning of terms to 
expand and assume a wider connotation than originally intended ; 
and the lack of a generally accepted classification. The first cause, 
as is shown by Holmes, really depends on the second ; since meanings 
inevitably expand and wander unless the terms are restrained within 
the bounds of a sufficiently rigid framework of classification. The 
question of classification is therefore at the root of our 
nomenclatorial difficulties; and agreement upon a classification 
by petrographers, or the enforcement of a classification through 
the influence of a sufticiently powerful and inspirmg teacher, © 
especially in the case of igneous rocks, seems to be a pre-requisite 
to the successful reform of the nomenclatiire. 
While not wishing to enter upon the extensive and involved 
subject of classification, it is desirable to remark that it is impossible 
for petrographers to wait until a genetic scheme of classification 
is evolved, particularly for igneous rocks. Furthermore, if and 
when a genetic classification is developed which commands general 
assent, it is extremely unlikely that it will lend itself to the 
detailed quantitative treatment that is so necessary for the 
purposes of petrographical research. In the meantime a grouping 
of rocks based on empirical characters susceptible to quantitative 
subdivision is urgently needed. Recent essays at classification 
have attempted to supply this need, but none have yet won general 
agreement amongst petrographers. There will be purists who will 
object that such groupings of rocks are not classifications at all, 
but indexes. But so long as we have an instrument that will enable 
us to describe and compare rocks in an exact and detailed manner, 
we shall be indifferent as to whether it is called a classification, 
a grouping, or even an index. Genetic classification will come in 
due time ; but it will be broad-based upon the exact data provided 
by the quantitative instrument. 
