Page:History of West Australia.djvu/395

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stalk-like pieces. The constituting minerals of this rock are plagioclase or triclinic feldspar and hornblende.

From the surface down to a considerable depth the diorite schists are usually altered, their feldspar being more or less kaolinised, and their hornblende transformed into a chloritic mineral. The removal of lime and sodium in form of soluble carbonate by percolating waters is the cause of this alteration. The outcrops of diorite schists appear therefore often as chloritic schists. Diabase schists occur in several places.

Porphyroides, or schistose porphyroides, form dykes, breaking through diorite schist country-formation. In such instances the dyke rock has assumed the schistose texture of the country rock. There is no contact alteration observable.

Striped ferruginous ribbon jaspers form sometimes precipitous declines of hill ranges. They generally indicate lines of fissures and dislocations. These jaspers are the result of a hydrothermal alteration. They usually adjoin the dyke fissures. The overheated waters forming part of the igneous dyke-magma have forced their way into the adjoining schistose country rock. Less resistance was offered along cleavage planes. By the introduction of silica, and partly re-crystallisation of the original rock material, the mineralogical constitution of the original rock was completely altered, and the schistosity maintained, if not more accentuated. In their crosscuts such jaspers show stripes of different colours, principally reddish-brown, grey, green, yellow and white. This metamorphosis does not extend far from the fracture, and in several instances the successive transition from ribbon jasper into dioritic or chloritic schist can be observed.

All the massive greenstones of the interior gold region belong either to the diorite or diabase group. Felsite porphyries are the youngest igneous rocks of this region, and are considered here conjointly with the greenstones.

Diorite and diabase, in connection with their corresponding schistose features, are the principal material of which nearly all the greenstone ranges and hills of the interior gold region consist. Porphyrites occur in dykes. They also form extensive banks, and low massive complexes of rounded hills. Felsite porphyry also forms dykes and banks.

.—Beds of greenstone and felsite tuffs, and also calcareous and dolomitic tuffs, are abundant and extensive. The greenstone tuffs form banks and layers of dense, and occasionally schistose, rock material, within which larger greenstone fragments are embedded. They are usually of a dirty greenish colour. The felsite tuffs consist of a sandy and ash-like material, in which spherulites, from the size of a pea to that of a hen's egg, are embedded. Cross-sections of such spherulites present successive spheres, which are less decomposed towards the centre. The central part in larger pieces consists mostly of felsite porphyry, and occasionally of a dark porphyritic rock. In some instances amphibolite, diorite, hornstone, quartz, feldspar, and hematite, in fragments of half-an-inch to an inch diameter, are completely enclosed in the centre of such sperulites. These tuffs have in all likelihood a similar origin to that of the volcanic tuffs of the present day. They were either hot mud ejections or scattered lava, ejected by means of steam and gas explosions which took place in the channels below the vents. The ejected material was scattered in form of volcanic ashes, sands, lapilli, and sperulites. The rock and mineral fragments, enclosed in the above-mentioned sperulites, were unfused particles in the ejected lava mass. Some of the tuff-beds show that they have originated as hot mud emanations; others, again, are in all probability thermal deposits. Calcareous and dolomitic tuff-beds were formed, and are formed even in recent times, within the great lacustral depressions. Their material is chiefly derived from concentrated mineral solutions. These solutions were meteoric waters, which have entered and passed through the silicate rocks. On their journey they have performed their work of rock decomposition, and have charged themselves with soluble mineral matter. When these mineral solutions have entered into the highly concentrated salt water of lacustral depressions, most of their lime and also magnesia became precipitated as carbonates. Such precipitates, intermixed with pulverulent gypsum, common salt, and detrital matter transported and deposited by wind, have formed extensive tuff-beds within great lacustral depressions.

The tuff-beds were formed by the aid of water. Their material is in most cases partially decomposed, and some of them are stratified.

.—In the northern parts of the interior gold region, gneisso-granitic elevations are sometimes overlaid by beds and cappings of ferruginous grits, sandstones and conglomerates, which hardly ever attain any considerable thickness. Similar conglomerates occur as contact formations between gneissic granite and overlaying diorite cappings. Those strata are bare of palæontological proof. Probably they are representatives of the Cambrian section.

Neogene sandstone-beds occur also within the northern portion of the gold region. Occasional rainfalls assume here already a more tropical character, and probably succeeding accumulations and evaporations of surface water have been the means of cementing loose sand deposits. Those sandstone-beds are more or less water-bearing, and therefore of importance in regard to water supply.

covers archæan rocks as surface stratum. It is generally the result of their accumulative decomposition.

is formed in situ by accumulative decomposition of palæozoic greenstones.

are formed in situ by surface accumulation of more resistive and bulky rock fragments during hydraulic or æolian denudation.

.—Fine grained, argillaceous, mostly reddish-brown coloured strata, which cover the largest portions of the flat, trough-shaped depressions within this undulating region. The bulk of their material is derived from elevations where æolian denudation is taking place. Sand and dust resulting from the general decomposition of rocks is taken possession of by winds. Under more favourable conditions, the former soon settles, forming stretches of so-called sand plains. The finer dust is carried to a much greater distance till it is retained between the roots of the vegetation, principally when there are rainfalls.

The material of which the löess strata are built was subjected in this manner to a natural dry-dressing process. These strata are therefore usually even-grained, and the grains are always sharp-cornered. In this place it may be mentioned that by careful observation of the löess surface it is possible for the traveller to determine the direction in which the nearest outcrop of granite is situated, and from the quality and size of the sand grains conclusions as to its probable distance can be arrived at.

During my early travels in this region this knowledge was of great use to me in the search for water; namma holes and native soaks being confined to outcropping granite rocks.

are of frequent occurrence along the shores of salt lakes.