The CrEOLOCiY and Physicx.-kaphy of the (Josnells Area. 
Potash felspar is always nuieli fresher than the plaj^'ioclase, and any 
alteration is by kaolinisation, not serieiiisatioii. As it always sliows f*'ri(l- 
iron twinning', it lias been deseribial as iuiero(dine. No orthoelasc' was 
recog'iiised in any section of a ^Tanite, Occasionally, thin striiy^'s of clear 
-quartz tliread the potash fidspar, forniinjj' an injection iuicrope<>niat ite. 
Quartz is found as ('tear ii'rains which always show wavy (extinction. 
Next to f{‘ls}>ar, it is the dominant luiiuu'al. 
I^iotite has the ]d('ochroic sclienu' N = light yellow, = Z = deep 
brown, but in many ro('ks it has changed to the gr('en \ai'iety which may bo 
interlaininated with cohmriess chlorite (^) and miiscovitt'. Tin* giaum biotite, 
too, may la* replaced and pseudomorphed bv W(‘aPly birefringent gi'een 
('hlorit(‘. 
Kpidote, if iiresent, usually forms coarsi* turbid graindes associated with 
biotite, as it does in the xenoiiths. Rocks containing this 1yp(‘ of cjiidote 
are, then, slightly hybridised. Epidote in th(‘ veins emanating from the 
■epidioritf's is colourless to (-anary yellow and chair. 
Short stumjis of ajiatite ari^ rare ('onstitiumts of the gi'anitiv They are 
larger and more ])rominent m completfdy sei'icitis(Hl rocks. 
Occasionally sericite lias partially recrystallised to muscovite, which is 
■develop(xl both as anhedi'a within the iiarent tilagioclase and as laths Ixdween 
the ]>lagioclase grains. 
An analysis was made of an albite-rich granite in which tin* albite was 
moderately sericitised (it is estimated that alxuit 20 jier cent, of tlu' albite 
has been replaced by sericite). The result is sliown in Table II. in which it 
is compared with other granites analysed from th(» Darling Ranges near 
Perth, and with two ‘bi\'erage^^ types. 
The main point of difference between the Oosnells graniti' and the other 
two granit(?s near Perth which have been analysed is its extreme pooi-ness in 
magnesia and lime. It is slightly poorer in alumina, and slightly richer in 
silica, titania and both iron oxid(*s. The total alkalis in all three rocks ai'c 
fairly constant, although tlnwt; is consid(*rabh' \ariation in tin* rela(iv(* ])ro- 
portions of potash and soda. 
01' Daly’s average rock ty[)es (1933, p. !)) Ihe Oosnells granite resembles 
most closely the alkaline granite (Table II., column 3), but is (h'cidefUy low 
in ferric oxide, a little low in magnesia, linu* and tlu' alkalis; but ricluu’ in 
silica and alumina. Johannstm-s ‘‘kalialaskite” (Table II., column b) has, 
like the Oosiudls rock, a low magnesia and linu*. content. 
From tin* mode of the (losiu'lls granil(* (dkibh* I., (*olumn 1), its alkali 
content can be calculated to be NTnO KjO 2.0%. The excess ]>otash 
found in the analysis must be dm* to the sericite* Hakes, and to a little potash- 
fels])ar in solid solution in the plagioclase. 
The ])Iagioclase of tin* gi'anite lu'obabty crystallised as a solid seHution 
containing up to 30% of |)otash r(*lsi>ai‘. The solubility of the two diminislu's 
rapidly with fail in t(‘mperatur(* (»Johannsen, 1932, p. 141), and the excess 
potash-felsjiar normally separates as antiperthite. In tlu* (losnells granite, 
sericite is found instead of p(‘rthitic inlergrowths : the potash-content of 
the plagioclase se])ai'at('d as sericite, which is stal)le at a low tem])erature 
and ((ispecially) under conditions of stress. Some I'ocks contain unsericitised 
plagioclase and much })otash-fels])ar, and many of th(‘S(* microcline-rich 
