Some Basalts from the North Kimberley, Western Australia. 
85 
■other, and are often partially micaeisod. They are associated with laths of 
clinopyroxone, so altered to chlorite that the composition of the pyroxene 
camiot be gauged. These laths are set in a base of dark glass with which are 
associated numerous areas of a(>pIe-groen chlorite, showing ultra-blue polariza- 
tion colours. Tt is this chlorite, pi-esumabh% which accounts for the high 
MgO content in the chemical analysis of [3770] (Table IT., No. 2). In some 
of the specimens, notably [3777], the felspar has been oxtonsively altered 
to epidoto, which occurs in irn'giilar areas through the sections. An analysis 
of this extensively altered specimen (Table II., No. 3) sliows that SiO.^, MgO, 
Na20, KoO and TiOo have beiai leaclu'd out of it, while the FeO has been oxi- 
dised, and a large (quantity of OaO has been introduced. 
In the absence of an analysis, t hese rocks y ould be regarded as basic pyrox- 
ene-andesites, but their chemistry indicat(‘s their basaltic charat^ter, and 
their affinity with the other rocks from this region. Specimen [3772 1 appears 
to be intermediate between these audesine-basa.lts and the two-pyroxene 
andesine-basalts of Group 2. The pyroxene in it forms colourless microphono- 
crysts, with (-h) 2V greater than 45‘^ so that it approaches augitc in composi- 
tion. The plagioclase is too mucli alttM’ed to secondary mica for its composition 
to be determined, but it shows a pronounced stellate arrangoim'ut, and the 
interstitial spaces are filled with a dark glass containing trichytic crystals 
of pyroxene, in [7240] the groundmass consists of a lattice woi’k of cijioritized 
pyroxene inicrolitos in a flark glass. In piac(>s this structure is replaced by 
granular calcite. 
Another specimen difficult to classify, but having affinities with this 
group is 17271], a dyke rock from near Mount Joseph. It consists of small 
stumpy laths of much altered andesine ophitically intergroYTi with areas 
of partially altered pyroxene, which shows a large optic axial angle, and 
appears to be augite. The glassy base is much altered, and patches of chlorite 
occur scattered throughout the rock. Associated with the chlorite are small 
areas of glaueophane, intensely ]ileoc]iroic from deep-green to pale-brown, 
pale violet, or nearly colourless. 
4. Microcrifslalline andesine-bamltt-i. 
f37.>9], from the Synnott Tableland, north-east of Camp F.B. 32. 
17230], a dyke at the base of Mt. Brennan, intruding Xullagiiie sandstone (Fitz- 
gerald). (Barqnhai'son suggests it mav l)e an interbedded lava or a 
sill.) 
[7240], from th(‘ Hann River, near its junction with the Barnett Riv^er. 
As will be seen from Figure 2, two of these rocks come from tlie south- 
east portion of the region, away from the main ai'oas of basalt, as ma}}ped by 
Easton, while tlie third occurs in the south-central part, associated with other 
basaltic ty])es, 
[7240] consists of minute laths of plagiocdase accompanied by small 
grains of iron ore, in an abundant groundmass of glassy material that has been 
altered to pah'-green chloritt;. Tlu; felspar laths lie at all angles to one another, 
with no SLigg(vstion of parallelism, and show extinction anghvs up to 12° in the 
symmetrical zone wdien twinned, and almost straight extinction whmi micro- 
litic, indicating that they coTisist of acid andesine, of a composition about 
Ab-Q. Small patches of (*alcite occur throughout the rock, in the glass ; anil 
calcite fills a iuiml)er of small amygdules in the rock, when it is associated 
with small amounts of radially fibrous chlorite and finely granular cpiartz. 
Similar granules of quartz occur through the rock in the vicinity of the amyg- 
dules. Oi'casionally an amygdule is coniydetely filled with chlorite. 
