IGNEOUS ROCKS OF TASMANIA. _ 267 
The United States petrographers are in general agree- 
ment as to a chemico-mineralogical basis of classification, 
and their groups do not differ from those already familiar 
to us. In 1897 a committee, comprising Professor Van 
Hise, J. S. Diller, W. H. Weed, H. W. Turner, and Whit- 
man Cross, was appointed by the Director of the United 
States Geological Survey, to report upon the nomenclature 
and classification of eruptive rocks for adoption by the 
survey. The following scheme corresponds closely with the 
committee’s recommendations : — 
Granite-rhyolite family. 
Syenite-trachyte family. 
Nepheline-leucite rocks. 
Diorite-andesite family. 
Gabbro-basalt family. 
Peridotite family. 
The well-known groups are all here, and the recommenda- 
tions indicate, as it were, the direction in which Anglo- 
Saxon petrographers may be brought into line. An 
objection which may be urged, is that a certain want of 
proportion is apparent if these divisions are to be considered 
as primary ones. 
I ought not to omit mention of a tentative classification 
of the rocks of New South Wales put forward in 1890 by 
Professor F. W. Edgeworth David. Its basis is the silica 
percentage, to which I have previously alluded. As a result, 
it groups together in one primary division.such widely- 
separated rocks as felspar and leucite basalts. With this 
principle of classification unnatural unions are inevitah'e. 
I wish to be understood in this matter of S10, per cent. 
It is of distinct value in sub-divisions, and ought to receive 
expression in such, but per se it is not the all-powerful 
key which we perhaps have been accustomed to consider it. 
The more intimately we become acquainted with eruptive 
rocks, the more definite grows the conviction that ne single 
element has governed the differentiation of their magma, 
not Si, nor Al, nor Na and K alone; their composition must 
be considered as a whole. If undue weight is laid on tke 
SiO, proportion, the petrographer is sure to go wrong. Take 
a concrete example, one not far from home. Messrs. 
Guthrie, Woolnough, and Professor David have lately 
described a remarkable phonolitic rock from Kosciusko 
with a large proportion of nepheline, but with a low SiO? 
per cent. (52-4 per cent.) The quantity of silica is lower 
than that of phonolite proper, and is, in fact, that of typical 
basalt ; but to follow this as a guide would be hopelessly 
misleading. The authors say—‘ The Kosciusko rock differs 
