19 
Fig. 19. 
Stratified Rocks beneath the Volcanic Series, Foriescue 
Gorge, Hamersley Range. 
though in certain places, such as Mount Ankatell they are closely 
allied to the augite-andesites ; while at Bamboo Creek on one of 
the tributaries of the Cooiigan River, acidic lavas, quartz felsite, 
or rhychte, occur near the base of the Nullagine Series. 
The steam-holes in many of the amygdaloidal lavas are filled 
with secondary minerals, partly chalcedony and partly calcite. 
'I'he wide-spread occurrence of these lavas and their associates, 
which are much more numerous and wide-spread in the far north, 
nearer to that great circle of fire which forms a festoon round 
Northern Australia, and the relatively few volcanic focii so far 
noticed, would seem to imply that fissure eru])tions played an im- 
portant part in the formation. This type of volcanism finds a 
jiarallel in the 200.0(10 square miles occupied by the Deccan Trap 
areas of India, and those extensive lava plains of Northern 
Queensland, which 1 have been privileged to examine. 
.As has already been pointed out, the Nullagine volcanic series 
lies very near the base of the formation, and the lavas from which 
it is made up vary in composition from basic to acidic. The precise 
cause of this differentiation, which seems to have produced what 
may be called a gradational series of rock types, has an important 
theoretical hearing, yet requires careful investigation. 
An imjjortant group of carbonate rocks occupies a distinctive 
and well-marked stratigraphical horizon in the Nullagine Forma- 
tion. and covers a very considerable area of country in the Barlee 
and Hamersley Ranges, and in the valleys of the Ashburton and 
Oakover Rivers. As developed in these districts the group nor- 
mally consists of magnesian limestones, which vary somewhat not 
only in chemical composition but in general ap]5earance. Several 
analyses of these have been made, and the limestones have been 
