Page:Handbook of Western Australia.djvu/33

Rh therefore possibly be more varied in character than is commonly supposed.

Of the Lake District more is known, but by no means as much as is to be desired. It presents an irregular undulating surface of granitic rocks, very varied, as is usual, in composition and structure, traversed by dykes of dioritic quartzose and schistose rocks, the general direction of which may be found to correspond to the normal lines already indicated. Masses termed indifferently trap or basalt by explorers, and cairns of granite, form isolated hilts and ranges, the relations of which have yet to be determined. Quartzose and schistose dykes in erupted masses appear to be most common in the South, and trap or basalt in the North.

Upon the undulating surface of the primitive rocks of this district horizontal strata of sandstone, but of no great thickness, have been deposited; the elevations are separated by broad irregular shallow troughs, the depressions in the surface of the primitive rocks, in which are deposited clays and sands disintegrated from them, which act as receptacles for water, and form lakes of greater or smaller extent, according to the amount of the rainfall, and which uniting in very wet seasons may have given rise to the report, once prevalent, of an inland sea. The lakes or swamps thus formed may probably cover one-third of the entire area of this district South of the sources of the Murchison river.

The disintegration of the surface rocks, even of the granite, from the quantity of feldspar contained in it, but more especially of the sandstone and schistose rocks, is effected with great rapidity both here and elsewhere in the Colony, and is not confined to wet periods, but is continuous from atmospheric action during the whole year; and it may be assumed that where sandstones