166 
Clarke and Williaims. — Geolofii/ of DarJing Ttange. 
-Mill Brook flow in tlioir u])por ])firts through wide shallow valleys 
111 the laterite-eovc'red Darling l\‘iiej)laiji, but on reaehing the 
s]o])es of the Helena Valley they have tin- same character as the 
wholly immature tributaries. The mature of Smith’s IMill 
Brook lies outside the Area. Piesse Brook is a mucli larm-r 
])ereiiiiial stream, whose grade is only a little steeper than that 
of the Helena; in other words it has excavated a valley nearly 
as pronouiUH'd as that of the Helena. Its ii[)per portion however, 
six mih's south of the Darlington Area, has the same mature 
character as the other streams of this group. 
(c) Beheaded tributaries — (Jreenmount and Hosking Brooks liave 
little ill cimimon except their lieheaded character. Greenmount 
Brook now has short immature insecpieiit south-flowing head waters 
fi'ilowed by a mile of meandering mature course running more 
or less parallel to the Helena. West of the Area it again becomes 
stee]) and immature. At tlie transition between head and middle 
Avaters is a broad flat Aviiid gaj), in Avhich, in a. deep railway 
cutting, is disclosed partially consolidated sandy alluAduni. About 
a mile farther from Pc'rtli the railwav, which is here folloAvinn 
up kSniitli’s Mill Brook, passes through a more extensive deposit 
of th(^ same nature. .It is iiroliable, both from the unnaturally 
mature character of the niiddle-Avaters of Greenmount Brook, and 
from the occui'iamces of allu\duni in geiiei'al alignment Avith the 
upper jiart of Smith’s Hill Brook, that the middle part of 
Gi'oeninount Brook is a lower })ortion of Smith’s Hill Brook, be- 
headed l)y a more actively corrading inseipient tributary of the 
Helena. Hosking Brook is a sliort Avatercourse, mature except at its 
source at an abrupt saddle, from, the other side of which a short 
immature watercourse drains into Piesse Brook. The course of 
Hosking Brook is straight, and in aligninent Avith the mature 
liortion of the Helena lielow hSmitli ’s ilill Ib’ook, Avith Avhich also 
Us contours harmonize so Avell that anyone looking up the valley 
tiom lielow ih)ya A\'ould sup],mse it to lie the main river valley, 
and would not siisjiect the existence of the narroAV gorge to the 
left. j\lr. d. JS. Hosking, avIio mapped this portion of tlie Area, 
atli'ilmtes tliis ai»]iarent case of river capture to the abnormal 
activity of Piesse Brook folloAving on the uplift along the Darling 
l-hiult; this activity lie ascribes mainly to Piesse Brook being- 
situated in a zone of niorc' easily corraded rock. This explanation 
must be tested by in \-estigat ion of the couiitr\' (‘ast and soutli of 
the Darlington Area and its publication sliould lie an incentive 
to fnrtliei- work. 
6 . of geological stnictHrcH on stream-dvvection.— 
(a) Jointing and shearing— In some parts, particularly along the 
