IGNEOUS ROCKS OF TASMANIA. 269 
scheme will not prove amply adequate to all requirements. 
I commend it to your criticism. 
It is being continually urged upon petrologists by strati- 
graphical geologists that a nomenclature is needed which 
can be applied without resort to laboratory analysis. In 
short, field geologists wish to be able to name a rock at 
sight. Where it is impossible to determine the nature of 
rocks in the field, I do not see how a field-term can be any- 
thing more than a temporary expedient. The dyke-rocks 
and the marginally modified plutonics offer great difficulty, 
and I know of no Linnaean system which will extricate us. 
Field geologists must not complain if petrologists, before 
labelling a rock with a name, try by-all the methods at their 
command to find out its mineral and chemical composition, 
the conditions of its consolidation, and its genetic relations. 
Nomenclature can then be discussed with some degree of 
confidence. Others wish to be able to name rocks without 
knowing their mode of occurrence, but F. Zirkel puts the 
case convincingly when he says that “ petrology is a branch 
of geology, not of mineralogy, and its problems do not consist 
in determining specimens from unknown localities, nor are 
they solved in that manner. A rock specimen is not only 
an aggregate of such and such minerals, with a given struc- 
ture and composition, but is also at the same time a piece 
of the earth’s crust, which has played a definite geological 
role in the place whence it was taken; if that role be un- 
known, one essential elemerit of a decisive diagnosis is 
lacking.’’* 
In the sub-divisions I have followed the usual -plan of 
giving to geological occurrence its legitimate expression 
under the terms plutonic, dyke and effusive. But these 
terms must not be used slavishly, or difficulties will be 
encountered. Moreover, the dyke-rocks are exceptional in 
that they are not mere intrusives, but have a chemical 
significance. Granitic, basaltic, diabasic dykes do not come 
within Rosenbusch’s definition of dyke-rocks, though they 
have the dyke form. Dyke-rocks proper are differentiation 
products of the magmas of plutonic rocks. Naturally, each 
group of plutonics may be expected to be accompanied by 
its own dyke group. W. C. Brogger proposes the term 
“complementary rocks” for types differentiated out of a 
common magma. + 
Some of the dyke-rocks, e.g., lamprophyres in France and 
tinguaite porphyry in Brazil, have been shown to occur in 
* Lehrbuch der Petrographie, F. Zirkel, 1898, vol. i., p. 840. 
*On the basic eruptive rocks of Gran. Q.J. Geol. Soc., 1894, 
p. 81. ; 
