Some Basalts from the North Kimberley, Western Ax’Stralia. 
83 
The plagioclas^ occTirs as laths which range up to 1 inni. iii lengtli, when 
they appear to be porphyritio, but they average about 0*3 nun. They sliow 
extinction angles as higli as 33" in the symnietrical zone, and are optically 
positive, so that they are labradorite, of a composition about Al')40. Small 
areas of chlorite, pleochroic from green to jiale straw, are of fr(‘quent occui*- 
renee. They show straight extinction parallel to a single, M'idely-spaced, hut 
perfect cleavage, so that tlioy appear to i*epres('nt altercnl biotite. Sometimes 
the chlorite is fibrous, with a plumose structure ; and sonK'times, as in [375()J, 
it appears to bo pseudomorphous after small crystals of olivine. The iron 
ore occurs as sporadic coarse, irregular shaped grains of ihnenite and ilmenite- 
magnetito intergroMths, moulded on the pyroxene, an<l oplhtic tov’ai’ds the 
felspar. Tht're is also a certain amount of interstitial fclspatliic base, which 
has been altered to soricite ; and occasionally, as in [3745], [37fi8], and [3774], 
there are intersertal ai‘cas w hich consist of a grauopliyric intoigrow th of ejuartz 
and orthoclase. The chemical analysi's, Table I., Nos. 1-4, suggest that the 
specimens showing these micrograjihic intergi-owths are slightly more* acid 
than those in w Inch they are lacking. The possible significance of this is re- 
ferred to later. The nearest analogmt to these rocks in Western Australia is 
provided by the dolerites and th(* (juartz dolcritos w hich intrude the Nullagino 
formations in the vicinity of the LT})por Ashburton and X"pp<u* tlascoyne 
Rivers, in the North-Western Division (Maitland, 1909 ; Talbot, 1920). An 
analysis of one of these dolerites is quoted in Table L, A., for comparison. 
In most of the specimens the piagioclase is considerably alter^nl, having 
been changed to soricite or saussurito, and more rarely to epidote, as in [3753], 
when tlio change is reflected in the chemical comjiosition of the rock (Table F., 
No. 5). The pyroxene has g(ni(u*ally resisted alteration, or become slightly 
chloritized. 
In one specimen, liow'ever, |37f)0J fi'om Synnott tVeth, tlie alteration of 
both felspar and pyroxene is more striking. The original piagioclase has 
been completely altered to Icmou-yellow epidotes, whicli retains to some extent 
the lath-like form of the felspar, and to alhitc and (piartz. The albite, w4iich 
is dusted witli epidote granukjs, 0 {'curs as allotriomoiphic crystals 0*2 to 
0*5 mm. in diameter, interlocked witli allotriomorphic, and sometimes idio- 
morphic, crystals of (piartz of similar size. The cpiartz predominates. 
Mnch of (he soda set free during the alteration of the labradorite to e]>idote 
lias attacked the pyroxene, and converted it to a soda-am|)hibole, presumably 
glaucophane, which is intensely pleochroic, with X = pale violet, V = deo]) 
violet, Z tieep blue-green. It shows a good clt^avage paralk'l to (HO), 
and extinguishes at 30° on this cleavage. Reumaats of tlie original pyroxene 
are sometimes enclosed wdlhin the glaucophaue. S<une of tlu' pyroxene, on 
the otlier hantl, is intimately intergrowu witli the yellow* opidotf'. The amount 
of glaucopiiane present is cuiisiderahly less than th(' prubahlo amount of 
pyroxene ])resent in the original dolerit(\ and the analysis of the nx-k (Table 
I., No. f>) suggests that the glaucopiiane is rich in iron, ami [lossibly magnesia. 
It seems probable that the augite of the original roek has altemd, in part at 
least to ejiidote, while the pigeonite has changed to glaucopiiane. A little 
pyrite has Ix^en introduced, together with \nins and patches of calcito. When 
the chemi('al analysts of tht^ lock is eom]3ai'ed witli the analyses of the h^ss 
altered rocks of Table I., it is seen tliat there is a considerable general I’esem- 
Filance, as would be expected, but that K^O, MgO and TiO^ have be(ai loaclied 
out to a eonsideraljle degree iluring the alteration, w hile Na.^O lias been re- 
moved in lessia’ ilegree. On tlie other hand. SiO^, f’aO and (’().> have been 
introduced. 
