252 
C. E. S. Davis. 
2. Low-Level LaterUe. 
Ntnu- the foot of the Darling searp, laterite crops out in places, west 
of all outcrops of Pre-Cainbriaii rocks. In a gravelpit just west of the, 
Mountain Quarry, the deposit grades from ill consolidated detrital material 
mixed "with large quartz boulders to more homogeneous laterite. Small 
'^breakaways” in laterite ha\'e l)(‘eu noteil (e.g., at a height of 300 feet near 
the south-west corner of the area, and at a height of 180 feet, west of the 
White Hock Quarry). The laterite may not be all detrital in origin. 
Near Ellis Brook, just west of the porphyritic e])idiorite, is a red, clayey 
deposit with incipient pisolitic structurr^ — perhai)s a partially formed laterite. 
The heavy minerals of this clay and of the porphyritic ejiidiorite were 
almost exclusively magnetite, so it may be suggested that here the por[)hyritio 
epidiorite (or some other rock of favourabh' composition and texture) is 
being laterised m situ. But all that can definitely be asserted is that, near 
Gosnells at least, the low-level laterite* is too discontinuous to mark the posi- 
tion of a step-faulted block, as suggested by AVoolnough (1919, p. 16}. 
H. Later Stiperfcial Deposits. 
These include silt, sand and talus slopes. A little silt has been de]»osited 
in the valleys of the larger streams, but it is not shown on tl \3 map. Sand 
occurs over the laterite on the gently-sloping ])lateau and over low-level 
laterite near the plain. Except for the lateritic rubble which covers such 
a lot of the area, talus slopes are small. 
TV. PETROLOGY. 
A. XenoUths. 
Although in the field these are much darker, some contain only a little 
mor(‘ biotite than does the surrounding granite, so that the two rock-types 
are very similar in section. 
Usually, the biotite is a green variety (N = light yellow Y — Z = dark 
green) Avith inclusions of opaque magnetite (?) aligned parallel to c. In 
particularly biotitic rocks, recognisable felspar crystals are rare, their ])lace 
being taken by masses of finely granular epidote. E])idote may he found, 
too, as coarse, turbid and red stained grannies associated Avitli biotite 
aggregates. Quaifz is subordinate to biotite and epidote. Sphene (in the 
form of scattered granules) and apatite are rare. Most of the xenoliths are 
these biotitic rocks more or less digest(‘d by the ]dagioclase- and quartz-rich 
granite. 
Potash fels|>ar is found in veiy few of the xenoliths. Plagd<>f'b^^^* i^^ 
'saussuritised and sericitised to a varying extent; and some individuals con- 
tain small columnar crystals of zoisite. Extinction angles measured on albite 
twdii lamellae indicate that the felspar averages Ah,.., Avith a range in com- 
position from Ab^„ to Ab^,.. In rocks containing heavily sericitised felsi^ar, 
the biotite has recrystallised to coarser green flakes. Sometimes, this process 
is incomplete and leaves the biotite intergrown Avith muscovite or with 
colourless chlorite (?). 
Occasional specimens contain a. broAvn biotite (X = light yellow, Y = 
Z = deep copper-broAvn). Such a rock (with granular epidote marking the 
place of felspar) is found at Statham’s. Clarke and Williams (1926, p. 
169) noted that such ^finotitic .segi’egations” exist, but they did not describe 
t hem. 
