AFaitlanI). — .-I nslraiiaii (rioloffy. 
229 
belong' to (HU- si'rii-s aiul ronclu'd tlu'ir ]n*i.-s',‘iit ])osition at about 
the same ji'eolom'ica] period. Tlie dykes tluH'(‘fore are of late Ter- 
tiary a^'(' and belong' t(t the same period as the volcanic rocks iii 
South Aiistialia and Vi(d'oria. 
In tlu' Xorth-West Division a reniarkabh* and very important 
feature is the al)undance of dolerite intrusions in the fonn of 
slieets oi- steej)]y inclined dyk(‘s. Tlu'se rocks liave a remarkably 
uniforin comj)osition and exhil)it litth‘ or no trace of recrystal- 
lisation or other si|L>'ns of metamoi'i)liism, and are '|)vactically in 
the same condition in which tlu'y oi-i^inally cong'ealed. Some of 
them extend across country in niori' or h‘ss straight lines for many 
miles and give rise to fairly cous]dcnous fc'utures standing out 
])oldly in the backs of tin* ridg('S, of which an ('xcellent exam])le, 
the Dlack Range, is to be seen in I’ilbarra. At tinu's these dykes 
and sheets invade tlu* st'dimenta.ry rocks along the ]>lanes of bed- 
ding and occasionally aiadi u]) tlu' overlying strata; whilst at others 
they form tapt'ring slu'ots running out into the lu'ighbouring sedi- 
ments in form not unlike a cedar tree, 
From the number and v(u-y larg(* area over which these dolerite 
intrusions extend it appears that theu* must have b(*en a huge 
reservoir of molti'U matter lying lu'neath the surface to the north 
of S. Init. which merely awaited tin* suitable opj)OJ'tunity of 
lisiiig to the surfaced 
In the fai' mndli, in tlie l\imberl‘ey Division, basic lavas and 
volcardc ashes occur in gr(‘at force. These lavas ap]»ear to have 
Hooded tin* valleys of the Ord and How Rivers and h'velled up the 
(h'pressioiis. with the exc<‘ption of certain kntfe-e<lged ridges of 
tlu* oldei- rocks which still ])rotriide above the genei'al level. On 
the Delia Riv(*r, just above what is known as the Gorge, is a dome 
or “]uiy” of basalt whii-h f(?rmed one of the foci from Avhich these 
lavas issiu'd. None of the volcanoes are still active, though the 
liot spring at Mount Wynne imints to the fact that the igneous 
activity has not yet been entirely snrjiressed. 
At the close (d‘ this g(*;)!ogical period a, great paj't of the 
sonthcrii coaslal ]»lain and the adjai'ont bord(*rs of the interior' 
j'lateau wa're raiseil abo\'e the S('a-l(*v('l, i)iit the full extent of this 
n])lift is not as yet (hdinitely known, but it is ('ertain that the 
sea. retiH'ated considerably beyond its ])resent shore lino. 
Paid of the Westc'rii Australian coast line is occipiied by a 
icck series des.iguated the Coastal Linu'stone. the uneipial weather- 
ing of wliicli has i-esulted in the formation of an extensive system 
of caves and grottoes which yet await exploi'ation. The coastal 
limestone eontains abundant fossil inollusca, identifialilc with tliosG 
at present living in Australian waters, and furnishes unmistakable 
