XXX. 
Charlks Austin Gardner. 
The superficial deposit known as laterite by Western Australian geologists, 
is extensively developed on the Darling Plateau, and occurs in many parts 
of the southern interior, either capping the higher elevations in the form of a 
more or less continuous cuirass, or superficial accumulations of pebble-like 
lateritic nodules with varying amounts of clay and sand. Laterite also occurs 
near the northern edge of the Hann Plateau between Admiralty Gulf and 
the Couchman Range in Kimberley, associated there with palm woodlands 
and a type of sclerophyllous woodland. Laterite also provides the edaphic 
requirement of the jarrah forest. In the interior it is frequently associated 
with an impoverished sclerophyllous association in areas of low rainfall, 
typified by the “ Wodjii ” type of country, certain Casuarina thicket associa- 
tions, and sometimes with small dwarf woodlands of Callitris. The lateritic 
areas are usually the most deficient in grass, but surprisingly rich in Goodeniaceae. 
Finally, mention should be made of the sand which is so characteristic 
of large areas of the south-west, in the far interior, and in the littoral regions 
between the Gascoyne and Ashburton rivers, and again between the De Grey 
and Fitzroy rivers. Within the south-west, on the plateau, it is often difficult 
to divorce the sand from the laterite as a distinct formation, since laterite so 
frequently vinderlies the sand. But in any case, it is on the sand that the 
shrub-heaths hold undisputed sway, whether the sand be the loose detritus 
of the coastal ]dain, the humus sand of tlie southern littoral, or the more 
compact yellow or red sand of the interior. Even the sand of the desert 
supports psammojfiiytes with structural peculiarities, enabling them not 
only to thrive in a hostile environment, but sufficiently plastic to create new 
forms. Finally, it is in the sand that the south-west elements have made 
their furthest advance into the interior. 
V. CLIMATE. 
The position of Western Australia with reference to its range in latitude 
(almost equally tropical and extratropical), its low elevation and uniform 
topography, together with the fact that it lies on the western side of the con- 
tinent, and is, therefore, subjected to the desiccating effect of the south-easterly 
trade-winds, are facts of fundamental importance from the climatic standpoint. 
Tliese facts account for the different climatic systems, and for the great arid 
mi<ldle region wliich represents tlie western prolongation of the desert of 
Onti-al Australia. 
Three climatic zones may be recognised : a northern area of summer 
rainfall of a monsoonal character, with a cool dr^^ season ; a south-western 
area of winter rainfall with a period of summer drought, and a vast central 
ai't^a of low and unreliable rainfall of no marked periodicity, depending en- 
tirely upon extensions of the climatic systems which dominate the northern 
and southern areas. 
1. THE CLIMATIC CYCLE. 
In order to understand the three climatic areas it is necessary to give 
a bri(T account of the operation of the systems. 
In January and February, the thermal equator lies over the north, where 
high temperatures are experiencetl, especially over the Pilbara and Hamersley 
districts, which at this time are amongst the hottest regions of the earth’s 
surface. The intense heat causes an inflow of cooler air from the tropical ocean 
—a monsoonal system which brings rain from the north-west. A lag ” of 
from four to six weeks which occ\irs between the atmospheric movements and 
the apparent solar movement accounts for the retardation of the season 
which normally extends from December to March. There are two pluvial 
foci : one where the Hann Plateau extends to the coast between the King 
Leopold Range and Admiralty Gulf, where the seasonal precipitation exceeds 
