264 PROCEEDINGS OF SECTION OC. 
ON THE NOMENCLATURE AND CLASSIFICA- 
TION OF IGNEOUS ROCKS IN TASMANIA. 
By W. H. Twe.vetTress, F.G.S. 
Government Geologist of Tasmania. 
PETROGRAPHY and petrology are two sides of the study 
of rocks; the study of the subject as a whole comprises both. 
For the last forty years, petrography, stimulated by the use 
of the microscope, has worked its way steadily to the front 
in this department of geological science. Rocks have been 
studied as mineral aggregates, and the resources of the 
world’s great opticians and of their scientific advisers have 
been taxed to the utmost in devising instrumental means 
for the optical analysis of rock slices. The delicacy of these 
modern methods compels admiration and wonder. Armed 
with these aids, geologists have devoted years of toil to petro- 
graphical work. The structure and mineral constituents of 
rocks from every part of the world have been elucidated and 
minutely described. The latter half of the past century 
witnessed thisreignof petrography. Field geologists brought 
up in an older school felt themselves out of rapport with the 
new methods, and looked askance upon them. They had 
some reason, for an exclusive reliance upon optical analysis 
begets unnatural views. But the devotion to petrography, 
the description of rocks, did good service. Petrography was 
the necessary stepping-stone to something higher, petrology, 
the science of rocks. The one lays the foundation upon 
which the other builds its generalisations. The pursuit 
of petrography inevitably led to the creation of an interest 
in petrology such as had never been witnessed before. Not 
only the microscope, but also chemical analysis and field 
observation have been pressed into service to arrive at a 
knowledge of rock magmas and their consolidation into the 
known groups of eruptive rocks. As a consequence, the 
_ study of magmatic differentiation, of the genetic characters 
and relationships of rocks, and of their classification upon 
natural lines, has, during the last decade, occupied the atten- 
tion of our foremost petrologists. One result has been to 
impart a healthier tone to the study by restoring to field 
observation and chemical work some of the consideration of 
which they had been unjustly deprived for a time by the 
fascinating pursuit of optical analysis. None of these 
methods can be divorced from the other without injury to 
our beautiful science. Under the glamour of the new 
