The (Geology and Physicgraphy of the Gosnells Area. 
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Metamorph ism, 
Metamorphk' eftects in the Gosnells sediments are very slight. Tonr~ 
maline has developed in the slates, and serieite may be recrystallised. In 
the sandstones, a little quartz rccrystallised at some stag’e as cherty lenses 
and as (juartz veins. But hematite has not been changed to magnetite, and 
no biotite has developed. Only slig’ht jointing has been produced. Argil- 
laceous sediments are very readih^ affectedi by lise of temperature, and the 
very slig’ht metamorphism of these rocks indicates that they were deposited 
after the granite Jmd cooled. 
E . Bas ic In trn si ves. 
1. TJraJitised Quartz Dolerite (EpicUorlte) . 
Hand sneeimens of this rock are grey to greenish-black in colour and 
are line-grained near the edge of the dykes, but medium-grained near the 
centre. They consist of white felspar set in a darker matte of amjdiibole. 
Essential minerals ])resent in thin section are felspar, arnphibole, ei)idote 
and ilmenite. iVIost of the arnphibole is a pale variety consisting of llaky 
aggregates or irregular plates, and is ('vidently a secondaiy mineral. It is 
referred to as ‘Walite.’^ A litth; Y>Gniary brown arnphibole is present in 
most rocks, and bluish, more euhedral arnphibole is produced by recrystal- 
lisation of the uralit'*. Both these latter types of amj)hibole are referred 
to as “hornblende.^’ 
In similar basic dykes north of Gosnells (e.g., in the Lower Cluttering 
area (Miles, 1938, p. 29)), uralite is derived from ])yroxene. The author 
has seen a section of a dyke near Dailington in which pyroxene has partly 
altered to a ]>ale green uralitic aggregate which farther from the parent 
mineral, has recrystallised to a. blue-gTcen type of hornblende. Although 
no relii's of the primary, high-grad(‘ parent mineral of the uralite have 
been found in the Gosnells area, it is believed that this uralite, too, is 
derived from pyroxene. 
Holmes (1928, p. 92) defines “epidiorite” as “A doleritic or basaltic 
rock in which the augite has suffered alteration to hornblende so that the 
rock (mineralogically) approaches the composition of a diorite.’' The writer 
has added the word mineralogically as, chemically, such alteration does not 
produce a more acid rock. 
Basic dyk('s neai* Perth with pyroxene entirely replaced by horn])lende 
have been desciibed in ]>i-evious literature as “epidiorite.” This has, how- 
ever, become a fhdd- and a sack-name and moreover has often been altered 
to “diorite,” even in g(M)logica] pu])lieations. It is best, therefore, to give 
1o the rocks a name indicating their genetic relationships, and they are here 
termed uralitised qmu-tz dolerites. ITalitisation must have been deuteric, 
for the Cardup sediments are so very low-grade that there could have been 
no regional metamorphism to effect the change. 
Fresh primary felspars are clouded and coloured brown, but this 
colouration is far less marked than it is in dolerites from Parlington and 
Lower Cluttering. MacGi-egor (1931) suggested that, were a basic intrusive- 
