Some Basalts from the North Kimberley, Western Australia. 
87 
No. 4) all the inialterc([ specimens that have lieen analysed are eitlier .saturated 
with res]:tect to silica, or nearly so ; and cJiemicaily, they have many features 
in common with the tlioleiitic basalts (Kennedy, 1933) or plateau-basalts (Wash- 
ington, 1922) as distintd from the olivine-basalts (Kennedy, 1933), as is shown 
by Table 111, in Avhieh the averaged analyses of the more or less unaltered 
North Kimberley rocks are compared with averaged analyses of the East 
Kimberley basalts, the tlioleiites of soutli-western Western Australia, and 
the theoretical tholeiite magma type. This resemblance is particular! \' marked 
witli the North Tvimberley dolerites. The basalts tend to be rather more an- 
desitic in c.om|)osition. 
Mineralogi{'ally, also, the North l\imberley rocks are related to the 
tlioleiites and their derivatives. They tend to form a distinctly (*alc-alkaline 
suite, grading towards andesites, rather than towards the more alkaline 
types which accompany olivine-basalts. 
As indicated, the North Kimberley rocks resemble, in theii* cliemical com- 
position, those of East Kimberley (Edwards and C’larke, 1940), but the varieties 
of basalt known to occur in the two regions cannot be matched. This is not 
surprising if the Noi'th Kimberley rocks are of Nullagine age, while the East 
Kimberley basalts are Cambrian, or even |)ost-C^ambrian in part. The rela- 
tive abundance of dolerite specimens in the collection suggests that this rock 
is the most mdespread in the region, and, since it is also the most basic found 
there, it may approximate to the composition of the parent magma. AVhether 
it bears such a relation to the basalts, also, must remain doubtful, for reasons 
given below. 
As pointed out liy Farquharson (in. his unpublished manuscript), the 
dolerites from North Kimberley bear considerable resemblance to the dolerites 
described by Maitland (1909), and by Talbot and Farquharson (1 920), from 
the drainage basin of the Ashburton, Cascoyne, ami Oakover Rivers, and the 
Hammersley-Ophthalmia Plateau, in the North-West Division. The sig- 
nificance of this resemblance is that the latter dolerites occur largely in the 
form of sills, up to 300 feet thick, intru<Led int(t flat-lying beds of Nnllagine 
age (the Carawine Limestones aa<i otliers). Jn view i.)f MaitlancUs ohser\’atioiis 
referred to above (Maitland, 1902), this raised the <|ue.stion as to whether some 
or all of the dolerites in the North Kimberley may not occur as thin sills, ratiier 
than as lava flows. If this should prove to be so, it may be found that the sills 
have undergone some degree of differentiation in situ, and that the (juartz- 
dolerite facies is a [iroduct of this differentiation. 
lalhot and Far(juharson (1920) showed that the dolerites in the district 
described hy tJiem were yoiinger* tJiaii the basalts in that I’egion, because 
dykes of dolerite cut tiirough sedimentary series with whicli the basalts were 
interbedded. This raises the further (juestion as to wliether the basaltic rocks 
of the North Kimberley may not also be of two ages — earlier, intei’bedded 
basalts, and later intrusive dolerites. Tliis may well l>e so, but, since there 
are no details as to the mode of occia*ren<'e of the specimens in the collections 
examined, except an occasional description of a specimen as “ from a dyke ” 
(and these occur in four of the five grou|)s established above), and in the case 
of one dolerite specimen [3771J a note ("laccolith ?”), it is a matter that 
must be left for future exploreis in the North Kimberley to decide. 
