Page:History of West Australia.djvu/396

 344

are frequently met lacustrine features.

are lacustrine formations, of which the last series can be considered as lacustrine alluvium.

is limited to banks of watercourses, and to sandy flats in which creeks disappear.

are secondary formations derived from the decomposition of greenstones, and accumulated on the surface during an æolian denudation.

1. .—In the geological structure of the West Australian plateau, the gneissic granites appear as oblong blocks of huge dimensions, some of them showing widths of twenty and more miles.

The archæan earth-crust, consisting of the above-named rocks, was broken through by numerous fissures of a more or less west of northerly course, and the crust blocks produced in that manner have gradually adjusted themselves into their present positions.

The West Australian tableland must have originated as an arch-shaped bulging of the archæan earth-crust. Lateral pressure (explained in geology as a result of secular cooling and contraction of the earth-spheroid) may have caused that effect by acting in an east-west direction.

At a later period this archæan arch gave way, separating during its downbreak into the above-described crust-blocks.

On the tableland surface, the archæan strata appear as oblong elevations, tending longitudinally towards west of north. Those elevations enclose the previously described flat, trough-shaped valleys, and their rise above the lacustrine and löess strata is either gradual or it takes place abruptly, forming rocky ravines and cliffs.

The stratification of the archæan rocks on the plateau surface is either horizontal, undulating, or slightly inclined, and accordingly they form elevated plains, or they indicate monoclinal folds.

2. occur in several distinct belts of country, of more or less parallelism, west of northerly longitudinal extent. These belts are hundreds of miles in length, and follow the courses of gigantic fractures in the archæan strata. Each of these fractures consists of many fissures or systems of fissures, through which the material of the palæozoic greenstones has risen in a state of aqueo-igneous or hydrothermal fusion, and within which fissures it became solidified by successive cooling. The fracturing of the archæan earth-crust, as well as the magma emissions, were apparently due to lateral pressure.

The palæozoic greenstones within the interior gold region occur in the following forms:—

(a) Schistose diorites, bordered on the east and west by horizontal or inclined strata of gneissic granite. They generally occupy enormous-sized fissures and spaces of V-shaped cross-section within those strata. Where shafts have entered those schistose diorites to depths of 200 and 300 feet, it has generally been found that the schistosity gives place to a massive and jointed texture. Along contacts with gneissic granites, those diorites enter in form of apophyses and wedge-shaped dykes, the former rocks, and they also enclose some of their detached portions.

(b) Dioritlc, diabasic, porhyritic, and also felsitic dykes occur within the schistose diorites, and also within the archæan strata. They are derived apparently from the same magma reservoir as the diorites in which they occur. In several instances the dyke has received the same schistose texture as the country rock.

(c) Coarse crystalline massive greenstones, forming rounded hills and hill-complexes, which point out strongly that they are igneous material, solidified by gradual and slow cooling within the interior of the earth-crust. They are solidified magma reservoirs exposed by denudation.

(d) Massive diorite banks and cappings, overlaying archæan strata. In cases of direct contact between such diorite banks and granite, the diorite assumes aphanitic, and the granite granophyric texture.

(e) Successions of massive and schistose diorite strata in the structure of hill and mountain ranges. The massive strata increase in number and size on relative higher levels.

The large fractures and fissures within the archæan strata have served as vents for enormous submarine emissions of aqueo-igneous magma, which became deposited on the bottom of a palæozoic ocean. The difference of temperature at different times and different places in the ocean, as well as the occasional quantity of emitted magma, gave cause to a faster or slower cooling, and, accordingly, to a fine or coarse crystalline texture.

Spaces of V-shaped cross-section, formed by the tilting of archæan crust-blocks along fissures, have received the first emissions of aqueo-igneous magma which rose through those fissures. Those spaces have also preserved from denudation large portions of the rocks, into which the magma became solidified. The process of solidification took place in a centripetal direction, and during its progress the solid magma-crusts were subjected to the continuously acting lateral pressure. A general schistosity and numerous fissures and faults within that rock-crust were natural results of that force. The lower magma portions, remaining for long periods in a state of aqueo-igneous fusion, were forced by the same pressure through newly created fissures upwards.

The schistose diorites within the interior gold region have apparently originated as such solidified magma-crusts, and the numerous dykes which they enclose are vents of the rising magma, which ultimately became solidified within, and forms now the present dyke rocks. Thus lines of probably submarine volcanoes have originated along distinct lines of fracture in the archæan earthcrust. During undeterminable periods denudation has destroyed the bulk of eruptive features, which were formed in a like manner; but still, to a careful observation, many of the remnant portions disclose an undeniable similarity to recent volcanic structures, in form as well as in material.

The ejection of huge masses of eruptive material has necessarily left large cavities underneath; subsequent fractures and subsidences of rock-crusts which have formed the roofs of such cavities, have caused depressions. Within some of the great salt lake areas series of such downbreaks are even now recognisable.

The differences between elevations and depressions must have been once very considerable in this region. Denudation, lasting through geological ages, has reduced the probably once gigantic elevations to the size of mere hills and hill ranges, and the denuded material was made use of for the gradual filling in of great depressions.

In recent times this work is performed chiefly by subaërial, æolian, and chemical agencies, and it is very probable that during the mesozoic era hydraulic agencies have chiefly served the same end.