270 PROCEEDINGS OF SECTION C. 
the form of effusive sheets. It seems likely, unless an 
unusual sense be attached to the word, that the term 
“ dyke-rock ”’ will disappear, and some such term as “ com- 
plementary rocks” will take its place. The anomaly 
alluded to deforms the symmetry of our classification; but 
our schemes must not aim at being more symmetrical than 
nature. 
A basalt or a peridotite rock in the form of a dyke cannot 
be considered as a dyke-rock, for it does not differ consta- 
tutionally from effusive basalt or plutonic peridotites. Those 
who subdivide eruptives as plutonics, intrusives and effu- 
sives, do not transfer an intrusive plutonic to the intrusive 
sub-division upon geological grounds, nor can they who dis- 
tinguish the dyke-rocks chemically consent to add to them 
by transferring to their ranks intrusive forms of normal 
volcanics. Geologically, two main divisions should be 
erected—plutonic and effusive. The dyke-rocks occupy 
an intermediate position, not because they are intrusive, 
but because they are differentiated products. 
Even the normal rocks present difficulties. Duiabase in 
Europe is both effusive and intrusive. In Tasmania we 
have no evidence of its being effusive, but since, like basalt, 
it is the normal product of the gabbroid magma, its home 
cannot be among the dyke-rocks. For the same reason, 
quartz-porphyry modifications of granite have to be kept 
among the plutonics. Such instances warn us against 
aiming at unnatural precision. 
GRANITE. 
This may be defined as a plutonic rock essentially com- 
posed of alkali felspars and quartz. The felspars are the 
most acid of their species. The alkalies are present in mole- 
cular proportions of about 7 per cent., and the ratio of these 
to Cais 2:1. The SiO, per cent. ranges from 61 per cent. 
to 75 per cent., and occasionally even higher, fallmg im 
hornblende and augite granites, and rising in muscovite 
granite. 
The sub-divisions of granite have givenrise to controversy. 
Biotite granite is perhaps the most common type, and many 
have felt that it should be regarded as granite proper instead 
of being called granitite. Professor Hogg has recently at- 
tacked the position of granitite. Zirkel and Rosenbusch 
are at variance on the same point. There is something 
plausible in the suggestion, but historical considerations 
count for something, and the time-honoured idea of granite 
is that silvery mica is one of its triad group of minerals. I 
