In the first place, viewing the geographical and geological features in
combination, the tract of country contained within the 114th and 118th
parallels of longitude, and the 24th and 27th degrees of south latitude,
may be considered as an inclined plane, the eastern edge of which has an
elevation of about 1700 feet above the level of the sea. Commencing from
the coast, the first 100 miles is almost exclusively of tertiary
sandstone formation, which the process of denudation has, in many
instances, converted into either stony or sandy tracts, rarely fertile,
except when subject to the influence of frequent inundation. This region
seldom gives rise to rivers or watercourses; the flat-topped ranges,
which are often found towards the eastern limits of this formation, do
not generally exceed 500 or 600 feet in altitude, and are only those
portions of country that have not as yet yielded to the waste of time, or
the constant action of rivers, which, rising in the higher lands more to
the eastward, rapidly abrade, and in their onward course remove the soft
and porous sandstone from their bases.
In the deeper valleys, towards the eastern edge of these sandstones thin
beds of oolitic limestone, containing numerous fossil shells,
occasionally occur; also gypsum and clayey shales, with other indications
of the probable existence of coal in the vicinity; following the series
appears a compact, fine-grained amorphous sandstone, having an almost
flinty fracture; this rock, in a few miles, gives place to granite and
gneiss, frequently broken up by the upheaval of whinstone and porphyritic
trap hills, having an elevation of from 100 to 500 feet above the plain.
As we proceeded eastward, the eruptive rocks became more numerous;
chlorite slate, veins of quartz, chert, and variegated jasper, frequently
forming the summits of the most elevated hills, while, on the general
level of the plain, are occasionally found thin beds of ancient lava.
The rivers, unlike most others in Western Australia, have nearly an even
fall throughout their entire length, amounting on an average to six feet
per mile; this, in a country subject to the sudden fall of almost
tropical rains, is what gives rise to the destructive inundations already
described.