Olakke and Wili.ia.ms. — (irolof/ti of Darlh'f/ Uaiujc. 
167 
of tho tlio courisos of small stvoams are ])arallcl 
to the major joints or to the occasional shear zones. 
(1)) Kelathe i-esistance of granite ami e|)i(liovite — The fact that 
ill so many jilaces ]»romineiit si>u!-s ha\A' a coia' or e])i(liorite coii- 
lirins Aui'oiisseairs oliservation thtit epidiorite is as a laile mori' 
resistant to corrasion tliaii granite, Tlun'e is however considerable 
variation in the rc'sistaiice to weatlundiig shown l)y various epi- 
diorites. So far it has not been jiossible to ascriln' this to vari- 
ation in structun^ oi* eom])osition. 
4. Hiflli l(V(‘l tcrracc.s of Helena }'(ille}/.— -'i'he form-lines on the 
maji (Plate XXIH), ami also the ])hotogra])h of the to]>ographic 
model (Plate XXII) show that the steej) slopes of the valley are 
in ])laces interrupted by tlattcnu'd areas, and that these fall into 
two series, one lying at about 4d0 feet, the other at about 2o0 
feet above sea h'vid. Thesi* are ])robal>ly the remnants of two 
successive [)eriods of long continued lateral corrasion, Avhen the 
river had i)ractically attained base U>vel. In other words, they 
rccoial t'wo long ]>auses in uplift aUmg the Parling Fault. In- 
d.istinct signs of similar terracdiig may 1 h‘ seen in the Canning 
Valley, and along the Darling fault scarja 
Ilf — (lEOLOGV. 
A. General. 
The areas under discussion are essentially comjiosed of graiiitic 
rocks; almost entirely massive in the Darlington, predominantly 
gneissic in the Koh-ystmu' Area. The granites and gneisses are 
tiaversed by a great iiuml.>er of e])idiorite dykes varying in width 
from a fraction of an imdi to a clmin or more, and traci’abh' in 
some instances for moi’e than a mile along their strike. Whether 
all these dykes are jiractically coeval is not ceidain. yiiear zones 
traverse the granite and some of the epidiorite dykes, and along 
some of the shear zones there has been widespread replacement 
by silica. 
B. Acid Rocks. 
1. (>ranlte . — The granilt' is a coarse-grained l)iotite variety, 
generally with ])or])hyritic iiiicrocline. In the Koleyslone Area 
it is more or less gneissic; in tlie Darlington Area no truly 
gneissic gi-anite lias been found. 
Mineralogically the rocks from the two areas are identical. 
The microcline plieiiocrysts average about half an inch in length 
