82 
A. H. Howards. 
PETROLOGY. 
In the liand specimen tlie basalts can be divided into two distinct vari- 
eties, a ^roup of relati\'ely eoarse-grainerl rocks, and a group of extremely 
fine-graine< I rocks. Examination of thin sections under the microscope 
shows that the coarse-grained rocks are a uniform group of ophitic two- 
pyroxene dolerites. The fine-grained rocks, on the other hand, comprise 
basalts of several varieties that can be distinguished mineralogicaily and 
texturally. 
1 . Two-pi/ro.ren e dolerites. 
[87.'>(>J,* from Camp F. li. .SI. Cbanilcy diver. 
137,71], from Camp F.Jk 32, Charnley Hiver (analvsed), 
^ [37-73]. from the Isdell River (analvsed), 
[37dOj, from Camj) F.H. 32. Syiinutl Creek, Chai-tilev Uiver (analysed). 
|37h2J, from a large hill l)etweeti CarnpH F.B. 37 and 38. 
[37(i7], from a "pocket" near Camp F.B. 47. 
j3/(i9], from a iiill near Crosslaml s \Vart. Rriiice Regent River. 
[3771], from 3 miles south-east of Mt. Lyell (analysed). 
[377.7], from Cam]) F.B. 73. 
[7234], dyke in granite, from the right-hand branch of Barker River. 
(72.7')], dyke in th(' Lennaid Ihver, 10 miles below Mt. Eliza. 
Similar, but with granophyric intergi-owths of quartz and felspar : — 
137.74], from Camp C. 5. fsdell River. 
|37t)S], from gorge l)elovv F.B. .72 (analysed). 
1 3774 ], from 4 miles .south-west of F.B. 00 (analy.sed). 
Jhis group comprises 14 of tin* 35 specimens forming the collection, and 
includes the freshest specimens. As will be seen from Figure 2, the localities 
from wliich the specimens come are widely scattered over the North Kimberley, 
so that there can be little doubt that this variety of basalt is one of the wide- 
spread types of the region. 
Iwo sub-varieties can be recognised, a slightly more acid gi’oup with inter- 
sertal patches of granophyric (juartz and orthoclase (quartz-dolerites), and a 
more basic one in whicli siteh granopliyric intergrowdhs are lacking. Apart 
from this minoi difference, the rocks appear to be identical. Tliey are coarse- 
grained (Plato I, Fig. 1), ami tionsist essentially of pyroxene and basic plagio- 
clase, with some ddoritized biotite, an^l iron ore. Tlie pyroxene is the coarsest- 
grained constituent, and forms crystals about 1 to 2 mm. across, wliich are 
in ophitic relation with tliv felspar laths. 
Tw'o varieties of jyvroxiuie are present, in intimate assodation. One 
shows a (-r ) 2\ of about 50'^, so tliat it is a diopsidic augite, wIiMe the other 
show^s a (-r) 2\ of about so that it is a pigeonito. The extinction 
angles show- a general maximum of 35'^, with an occasional 4tF. LTsing the 
data givem by Deer and Wager (1938), this suggests that the compositions of 
the two pyroxenes are a]7proximatdy as follow^s : — 
Augite d MgSiOg 55 
l^FeSi 03 = 10 
f CaSiOg = 15 
Pigeonite d Mg8i03 --- 75 
l^FYSiOg =:^ 10 
1 
Numbers in brackets [ ] refer to specimens in the collection of the Geo- 
logical Survey of 7V.A. 
