AND PtlYSLOCAlAPaY OF THE OOSNELLS AkeA 
201 
A little of this hoinbleiule has been converted to biotite. However, 
aiiothei- recrystallised specimen (about three feet from the vdgv of a dyke/ 
■contains much biotit(‘ so that it resembles biotite e]>idiorites, described in the 
next section. 
A few V(‘ins of calcite, up to 3 cm. wide, occupy ])rominent joint cracks 
in the dykes. Some pyrite, too, has crystallised with the calcite in and near 
the veins. The rock surrounding- calcite veins is very low Ki’ade: it consists 
of pale green chlorite, (luartz and turbid patch(‘s of calcite. 
Occurrence of t;^dcm/.~The author has seen two loose spe<*ime 7 is of 
galena from Gosnelts, l)ut, in spite of a long- seai-ch, none itj situ. One, from 
a dump at the Blue Hock (tjuany, contains a mass of galena about 1 cm. 
in diameter, set in coarse, strained and slightly granulated milky (luart/. 
According to (luarrymen, it came from a mass of ([uartz adhering to tlu* 
granite at the dyke contact on the west wall of the north (|uarrv, but no 
galena-bearing quartz could be found in situ. 
The other specimen, from the White Hot k (^)uarry, was collected by 
S. E. Terrill early in 1935. Now that so much fuj-ther quarrying has betn'i 
earned out, it was inipossible to i*ecognise tin* dyke from which it came. 
His spe('imen is a hue-grained, rather schistose, dark rock, with fine dis- 
seminated grains of galena and pyrite. It eontains a Iilth‘ residual green 
hornblende and biotite, but the main ferro-magnesinn is ehlorite. Other 
minerals present are clear recrystallised albile, very subordinate epidote and 
a quartz vein containing some calcite. 
Qliis occurrence ot galena at. Gosnells, althongb on a veiy small scale, 
is interesting because the galena is d(>finitely associated with the uralitiscd 
quartz dolerite or one of its derivatives. It throws light on the genesis of 
the galona-sphalerite-(iuartz veins of Armadale (Prider, 1941, [)? 51) and 
silver-lead deposits at Mundijong (Esson, 1927), and conhrms Vridcr’s sug- 
gestion that they are genetically related to the basic rather than to the 
granitic magma. 
2. Bio t ite E puli o r Ue. 
A dyke, 19 feet wide, exposed only in the sonth(*rn coi-ner of the White 
Rock Quarry, is the sole rock found of this type. It contains many calcite 
veins and numei-ous small brown rounded biotitic patches. A section cut 
from the centre of the dyke shows that the }m\c liiue-green liornblende has 
been largely altered to brown biotite— the brown palelies consist of a decus- 
sate aggregate of biotite flakes, Recrystallised albit(‘ is common. Near its 
edges, the dyke consists of a line-grained mass of green biotite Avith sub- 
ordinate epidote and leiu^oxene, and clear felspar is rare. 
These rocks are similar to those described by Prider (1941, |>. 46) from 
Armadale which, being chemically very rich in potash and poor in lime, re- 
semble the chlorite-albite epidiorite and, probably, the porphyritic epidiorite 
(4. below). But the Gosnells rock seems to be a derivative of the iiralitised 
quartz dolerites, altered by end-})hase ])otassic solutions. 
A narrow epidotit' biotite epidiorite dyke (dve inclu^s wide) in the 
White Hock Quarry is Hnnly welded to tlie granite. It consists of epidote and 
(especially near its margin) of green-brown biotite. Microscopie veins of 
biotite and chlorite penetrate the granite for at least Vo cm. both between 
and through its constituent crystals, and cause a bending of the albite twin 
lamellae of the felspar. In composition, this dyke is intermediate between the 
biotite epidiorites and the biotite-epidote veins. 
j 331/42 
