The Geology and Physiography of the Gosnells Area. 
257 
quartz voinlots. Oec-asionaliy the twin lamellae of the feis])ars are bent; 
otherwise they are not altered. Quartz, as it is not unduly strained, must 
have been introdneed after the stresses operated to deform the felspar. Other 
specimens of g'ranit(% which can he seen in hand-siiecimen to be altered, 
contain felspar in various stages of replacement by s('ridte. So rock types 
are known, in the waihrocks and in the tiuartz masses, whicli are transitional 
between granite and pure massive (]uartz. 
Jt has been held that the (piartz “l>low’' is merely recrystaliised (|uartz- 
ite xenoiithic in the granite. But there is no trace of bedding in the “blow"’ 
(except possibly the sericitic bands found in the Mountain Quarry). It is 
unlikely that such a large s(ade structure would be obliterated elsewhere. 
Recrystallisation, too, shonhl j)roduce a ma^s of even-grained (juartz and 
muscovite, ajid no stockworks of (]Uartz threading the surrounding granite. 
Thin sections from the ^‘blow’’ and from narrow (juai'tz veins resemble 
one another. Both contain e\idence of replacement, and because so many 
transitional rocks are found, it is concluded that all these masses (including 
the wi<le “blow’’) were formed by liydrothermal replacement. 
Similar (piartz veins (both in Avidth and nature) occur near the Great 
Bear Lake, Canada. The Avall-rock (a granodiorite) is altered for distances 
of up to 1(10 feet froin the edge of the <puirtz \'eins. Such alteration 
(Purnival 1935, j), 855) consists of the replacement of felspar by secondai'v 
minerals (chiefly s(‘ri<*ite), followed by r(q>laceinent of original and secondary 
minerals by (piartz. Piirnival concludes ([). 859) that the hydrothermal 
solutions passed along faults of great persi.stence and disi)lacement along 
AA^hich the ro('ks were severely fractured over Avidths of up to 1,000 feet. 
Similar faulting in the massive granite near Gosnells, although it cannot yet 
be proved, is luwertheiess possil)le. The (piartz Avas i)robabIy derived from 
■end-stage solutions from the granite, as no other acid igneous rock has been 
found in the district. 
The long (piartz vein north of stream A contains small scattered crystals 
■of pyrite Avhich weather easily to make the I'ock appear \-ughy and to stain 
it reddish-brown, gi'een and yelloAV. Iron oxide has been deposited ])y ])erco~ 
lating Avuiter in many ])lae(\s in the adit, AA’hieh cuts through the AX'in 90 feet 
beloAV the surface. 
Veins 1 (in. wide, rich in blu(‘-grey tourmaline, cut the (juartz of the 
White Rock and Mountain Quarries. The quartz of these veins is eolourh'ss, 
limpid and coarse-grained (the grain size is up to 1 cm.) Imt excessively 
strained. The tourmaline is in the form of wisjis or rods, aggregating to 
an irregular Hlirous mass Avith pleochroic sclieme X — deej) blue, Z = jmle 
broAvn. The r('fracti\’e index e = 1.628 ± 0.003 indicating a tourmaline 
about midway b('tA\T*en (Iravite and schorlite (Winchell, 1!)27, )>. 246). 
Simpson (1931, p. 141) describes the oecuiTcnce of dravite in many places 
near Perth, and mentions that “In addition scliorl has Ixnm det(‘cted in small 
quantities at Gosnells and Cardup.” 
D. Cardiip Serieft. 
1. Arl-ose and Slate. 
Being intimately associated in the field, the two are discussed together. 
Arkosic hands and lenses in the slate contain pebbles of (piartz up to 
5 cm. long, set in a mass of smaller grains of rounded quartz, more turbid 
felspar and (in Aveathered specimens) white kaoHuite (?). Some slaty 
mat(‘rial is found in the arkosic patches. 
