The Vegetation op Western Australia. 
XXIX. 
cause not only do they support their cliaracteristic vegetation, but because 
they have been of importance in plant migrations. The salt pans provide 
their own halophytic associations. 
IV.— GEOLOGY AND SOILS. 
Apart from the climatic factor, the edaphic factor is the most important 
in j^roviding the particular requirements for individual plants, and in de- 
termining the characteristics of plant communities. The nature of the soil 
is dej:)endent upon the original rock formations which are fundamental in 
determining soil formation and soil characteristics. 
The geological fundament of Western Australia is a Precambrian com- 
plex, consisting mainly of metamorphic rocks and granite. The granite 
rocks are exposed in many parts of the territory, particularly in the south, and 
especially in the Darling and Porongorup Ranges, and parts of the interior 
of the plateau where they usually occur in the form of convex bosses or tors. 
Wherever the granite is exposed, or lies close to the surface, it exerts an in- 
fluence on the plant life, either because of the immaturity of the derived soil, 
or by reason of the water which it collects. Especially in the arid interior, 
the flora of the granite rocks stands in marked contrast to that of the sur- 
rounding country. 
Superposed on these basement rocks are many sedimentary series. Of 
these, the oldest is that knovm as the Nullagine. The rocks of this series are 
freely exposed in the Kimberley and Pilbarra districts, where they form the 
most prominent physical features of these areas. In the Kimberley district, 
on the Hann Plateau, and on the Antrim Plateau, the base of the Cambrian 
is composed of a series of basalt “flow's.” The occurrence of basalt in the 
surface soil determines a distinctive type of savannah or savannah-w'oodland 
in which the species of Themeda, the “ Kangaroo-grass,” is dominant, while 
the indicative prevailing tree is Eucalyptus Spenceriana. The “Nullagine” 
rocks, on the other hand, support on their derived soils a type of steppe or 
impoverished savannah in w'hich Triodia takes a prominent part in the physi- 
ognomy. 
The Devonian strata, typified by the Oscar and Napier Ranges, are im- 
portant because of the more richly varied flora w'hich they support. In the 
Napier Range we find Melia common, and several species not found in the 
neighbouring soil types. The number of endemics here is interesting. 
The Permian rocks of the Gascoyne-Ashburton divide appear to be im- 
i:»ortant in more respects than one. The area of tlieir occurrence here co- 
incides with that part of the Mulga-bush formation in wdiich the true Mulga 
(Acacia aneura) is either rare, or entirely absent, and the general physiognomy 
is more reminiscent of the Pindan country, a type w'hich is common along 
the low'er Eitzroy River, and paits of Dampier Land. The hard-pan soil, 
so typical of the true Mulga formation, appears to bo (juite absent from these 
areas. 
The rocks of the Kainozoic Era are associated wflth characteristic featiux's 
o^ the vegetation. They occm as sedimentary formations in many parts 
of the w'estern and southern littoral tracts, extending as far north as Vlaming 
Head, and on the south coast as far as Eucla. Between King George’s Sound 
and the Phillips River they occur in the form of siliceous sediments, and 
sup])ort a well-dif£erentiated flora ricli in many endemic IMyrtaceae and 
Proteaceae, mostly in the form of heath or low thicket formations ; finally, 
in a calcareous form they provide the basis for the phytogeographically distinct 
area of low salsolaceous shrub-steppe that characterises the Nullarbor Plain. 
