442 H. S. Wllbourn—Alteration of Granitic Rock. 
hills, on which is a beacon used by the Trigonometrical Survey, 
stands about 300 feet above the level of the surrounding country. . 
The general trend of the hill range is in a direction of S. 60° W. to 
N. 60° E., and there are several hills which branch from the main 
axis, usually more or less at right angles toit. The eastern half of 
the range over a distance of about half a mile only need be considered, 
as there are no rock exposures on the western half. 
The framework of the hills is seen to be made up of a white, 
fine-grained rock, containing small iron-stained cavities, which in 
its usual weathered state very much resembles a fine sandstone. 
Examination under the microscope shows it to be a very fine-grained 
igneous rock, consisting essentially of quartz and mica, sometimes with 
felspar, and nearly always with abundant blue tourmaline. In one or 
two places hard fragments of silicified shale were found lying on the 
hills, and one exposure showed patches of shales included in the white 
rock. It is doubtful whether these are shales of the Raub Series, 
which have become non-caleareous through the ordinary processes of 
weathering, or whether they belong to the younger Gondwana Series, 
consisting of shales interbedded with quartzites and sandstones. The 
boundary-line between outcrops of the two series lies in the locality, 
but its exact position cannot be traced. From the evidence of the 
dislocation of occasional quartz veins which penetrate the white rock, 
it is seen to have undergone faulting, and this is confirmed by micro- 
scopical examination of the shales. However, the amount of throw 
of the largest fault observed is only a few inches. 
A large outcrop of granite is situated to the south-west, about two 
miles from the Trigonometrical Survey beacon. 
In a hand-specimen the hard unweathered rock is seen to be very 
fine-grained. It is homogeneous in appearance, except that it contains 
dark-blue bodies, usually not bigger than a pea, sporadically scattered 
throughout the rock. They look very much like blots of blue-black 
ink on white blotting-paper. Under the microscope the rock is seen 
to consist for the main part of quartz and muscovite. The quartz 
occurs as irregular grains of a uniform small size, one separated from 
another by an aggregate of still finer grain, consisting of quartz and 
muscovite. There are a few flakes of yellowish mica of considerably 
larger size which contain tiny inclusions. Some specimens of the 
rock seem to contain no felspar at all. An attempt was made to 
separate felspar from a crushed sample of the rock by using a solution 
of mercury potassium iodide of a density such that orthoclase would 
float and quartz would sink. Some specimens proved to contain no 
felspar at all, others contained a small quantity of orthoclase. 
With still other specimens there is no need at all to use a heavy 
liquid in order to determine the presence of felspar, for small pheno- 
erysts can be seen: under the microscope. These can just be 
distinguished in the hand-specimen. They are of quartz, orthoclase, 
and plagioclase with extinction angle corresponding to oligoclase. 
The rest of the rock is made up of a mosaic of quartz and felspar with 
occasional flakes of yellowish mica, which contains inclusions. 
The quartz grains in the quartz-felspar mosaic are of the same size 
as those in the quartz-mica rock. The flakes of mica in the former 
