IGNEOUS ROCKS OF TASMANIA. 265 
power which the microscope had put into the hands of 
students, there was a danger of rocks being studied apart 
from their natural surroundings, and considered too exclu- 
sively and artificially as mere mineralogical and cabinet 
specimens, instead of integral parts of the earth’s crust. 
Unfortunately, petrologists are at variance in their 
principles and systems of nomenclature and classification. 
I am very sure that much of this variance is due to the 
differing histories of the science in different countries. 
English, French, and German observers have approached 
their subject from different standpoints, and international 
agreement has consequently been made more difficult. A 
further source of difficulty is the intrinsic nature of the 
subject. The eruptive rocks are aggregates of minerals 
which have separated out from eruptive magmas under 
varying physical conditions, and they share with organic 
groups or species the disadvantage (from a classificatory 
point” of view) of passing the one into the other through 
connecting varieties. New types are turning up with 
startling frequency, and the old simple nomenclature no 
longer covers the ground. These discoveries sometimes 
prove disconcerting, for they impose upon us the duty of - 
re-considering and often of re-adjusting our views. 
In advocating a few years ago, with Mr. Petterd, a system 
of classification for our Tasmanian igneous rocks founded 
upon varying acidity or basicity, it appeared far and away 
ahead of systems based upon structure, mineral contents, 
age, or geological occurrence. In that it rests upon a 
chemical basis, it still appears to me to possess advantages 
over structural or mineralogical systems. Without com- 
mitting my’colleague to my present view, I may say that 
the advances in our science during recent years have led me 
to recognise deficiencies in the old scheme. I venture to 
think that it fails in assigning the proper values to chemical 
relationships and differences. By reason of the importance 
it attaches to the silica percentage, it severs related rocks 
like granite and syenite, and at the same time associates 
unrelated rocks like elaeolite-syenite and normal syenite. 
In nature, the former pair blend together often enough in 
the same mountain massif, the latter never, for they ap- 
parently represent distinct magmas, and their respective 
dyke-rocks and effusive equivalents have their own dis- 
tinctive stamp and character. The dyke-rocks of elaeolite- 
syenite, for instance, will be tinguaites, not minettes, and its 
effusive product is phonolite, not normal trachyte. I look 
upon a system of classification as faulty if it neglects to 
take account of these important facts. In nature, too, I 
