The Geology or the Darling Scarp at Ridge Hill. 
109 
t'pidioi'ite dykes which is ovei’lnin ui the extreme south-easi comer by 
[he hio'h-level laterite. To tlie north and west oi' this line tlie surt'ace is 
covered by younger sedimentary rocks — a thin series of ferruginous sand- 
stones and conglomerates — which, in the northern dissected jnii't of the 
area, can be seen lying uncouformably on the Pre-(hi.mbriau roc'ks. This 
I ferruginous sandstone series is in turn overlain by a thin crust of laterite 
and is boumh'd to the west, by sandplain country which sloi>es gently and 
luiiformly down to the coastal plain still fartlicr west. An attemjjt has 
tieen made in the course of tin* matii>ing to differentiate btd.ween actual 
I outcrop of the fei-ruginous sandstone series, the detritus (talus) derived 
' from the weathei’ing of this series, the sandy and pehhly soils overlying 
I the ferruginous sandstones and low-level laterites, and the sandplain 
I country underlain by yellow sand. The areas occupied by these various 
! formations are indicated on the accomipanying geological map (Plate 1). 
! 13. THE PliK-CAMBlUAN ROCKS. 
These include granites, sheared granites (serieite scliist), epidiorite 
and quartz veins. 
( 1 ) Tiu' (irauiies ar(' the basement rocks and form a complex of two 
main types— a coarse-graimul pnrphyritie tyi)e with a slightly gneissic 
slmcture, and a tiner even-grained lyi)e with no trace of handing. In 
addition end-phas(> ])egmatites (griiidiic mierocline pegmatites) are also 
to be found. It was found iiupos.sihle to map the tW(J different tyjies of 
granite setiai-ately hut tin* relations between the two can he clearly seen 
in the freshly exi)osed sui‘fac(‘s in Stathams (juai'ry. In the south-western 
corner of this (piai'ry large angular xeuolithic blocks of the coarse-grained 
])n]’phyritic ami slightly gneissic granil{‘ occur in the massive liner-grained 
granite, lliiis inilicaling that the latter is the younger. 
The younger of these two granites exposed in Stathams quarry is very 
similar to the lounger Granite of Canning Dam which has l>een fully 
(h'^crihed in an earlier i)aper ( Pri(h‘r, 194"), p. 142) and no I'ui'ther peti’o- 
graphic details arc* retiuired here. Ther(' is, however, some dilference 
between the older granite of Statliams and the hybrid gneisses (Older 
Granite) of Canning Dam — the Older Granite fi-orn Canning Dam generally 
has a migmatitic structure and is free from mierocline whereas tlu> older 
granite ])hase at Stathams has 710 migmatitic structui'e and contains 
ab 7 jndant mici'odim*. It is similar in mineraiogical composition to the 
younger granite hut ditfeu's fi'om it in Ix'iiig much coarstw-grained and 
.'lightly gneissic. Plumocrystal microcli 7 ic in well-shaped crystals to one 
cm. or mo)-e diameter is an ahumhuit constituent and the periphcT’al zone 
o 7 i(' or two mm. wiile of such. ])iu‘no<'rysts consists g(merally of inicro- 
i•egmatit(^ 'Phe microclines contain iiicliisions of sei'icilised oligoclase and 
clotted hiotite Hakes which are the rno.st ahumhiiit coiistitiieiits of the 
liToundmass. The s’ig'ht gneissoid stiaictiire of these granites is due to the 
sub-parallel How orientatioii of the mim’ocliiie pluuiocrysts. In soine places 
this primary How structure is very well developed, e.g., at 17 chains 
.south-east froiii the centre of Stathaiiis (piaiuy it sti-ikes oo®. 
In view of the close similarity in minei alogical com])()sitio 7 i of the 
tine-grai7ied gi'anites and coarse-grained gneissoid granites of the xeno- 
liths it aiipears most i)rohahle that they both belong to the same magma 
vthe Younger Gi-anite magma) atid tliat tlu‘ xenoliths 7‘epresent an earlier 
