August, 1938 The Queensland Naturalist 
105 
ally up to an inch in length. The rock is not holocrystal- 
line, as there is a certain amount of glassy base. The 
minerals present in order of abundance are plagioclase, 
hornblende, orthoclase, biotite, quartz, magnetite, sphene, 
calcite and apatite. 
The rock has been very fully described by Jensen* 
who also gives a complete chemical analysis. This latter 
shows the rock as containing a little less than 60 per cent, 
of silica. 
The northern part of the intrusion at Point Arkwright 
on the foreshore is a fine grained pink aplite, which quite 
clearly has intruded the porphyrite. It is holocrystalline, 
and the order of abundance of the minerals) is, pink ortho- 
clase, quartz and a small amount of hornblende, which is 
often altered into chlorite. Frequently the orthoclase 
and quartz are intergrown owing to their being present 
in eutectic proportions. It is named by Jensen a grano- 
phyric aplite*, and the name is well merited. 
In comparison with the Noosa. Head intrusions we do 
not find the dyke-like bodies so characteristic of that 
place, also at Noosa the diorite mass is much more acid, 
but there is nothing corresponding to the massive grano- 
phyric aplite near Coolum Beach. 
*Proc. Linn. Soc., N.S.W., 1906, Pt. 1, pp. 122-124; also p. 169. 
VOLCANIC ACTIVITY. 
At Mt. Coolum , which is some 600 feet high, the vol- 
canic plug of comen dite is in the nature of a huge flat- 
tened dome. Like the several plugs of the Glasshouses, 
the Coolum mass is destructive and is a bluish grey in 
colour, fine-grained with dark blue-black specks of amphi- 
bole, probably riebeckite. 
Jensen** has described several specimens from about 
Coolum, and under the microscope the rock is finely holo- 
crystalline, is made up of idiomorphic, lath-shaped 
crystals of sanidine, deep blue almost black rods of soda- 
rich hornblende (riebeckite), and small masses of inter- 
stitial quartz. 
On analysis the rock yields somewhat unexpectedly 
nearty 75 per cent, silica, and is rich in alkalies, having 
4.77 per cent. Soda and 3.99 per cent. Potash. Its specific 
gravity is 2.70. 
To one familiar with the alkali rich Kainozoie vol- 
canic plugs of the Glasshouses and elsewhere in this area, 
the Coolum mass clearly is one of the family. Columnar 
structure is quite common in the mass and fluxion struc- 
ture is displayed well, quite frequently. 
*Op. cit. p. 126. 
**Op. cit. pp. 155-156. 
