The Geology and Physiography of the Gosnells Area. 251 
F. Edge of the Pre-Cambrian Rocks. 
The western boundary of the Pre-Carnbrian roeks cannot be drawn 
accurately. Some small, isolated masses of j^i-anite ar(' found several chains 
west of the edge of the continuously outcropping granite. These may be 
outcro]>s, and may be boulders. Granite fragments are found in a well, 
20 feet dee[», 200 yards W.N.W. oi' the jMountain Quarry, far from any 
granitf* outcrop and 100 yards west of the nearest quartz outcrop. Near 
the 8ealorth Home, a (|uartz mass extends far west of otluu' outcrops. But 
in spite of these irregularities, and other irregidarities due to laterite in the 
south, the boundary runs faii'ly straight in a north-soutli direction. 
Outcrops of (lie Cardup series are very discontinuous. They are found 
next at Kelmscolt, dVo miles farther south (Honman, 1912, ]>. 63). They 
may exist, however', under the laterite rubble which is so common in the 
south of the Gosnells area. 
Tei’tiary beds, found in Irores near Perth, are probably developed west 
of the Pi*e-Cambriaii rocks. A sulr-artesian boi*e {dejdh 172 feet) was put 
down near' Gosnells in 1872 Its exact location cannot be ascertained — it is 
mentioned by Brown (1873, p. 10), who staled that (among other things) 
fragments of lignite had been reportral from the borr*. Evidently the bore 
Avas put doAvn entirely in tlu' youngm' rocks west of tlu' P]*e-(bunbrian : 
Sini 2 )son (1916, p. 173) called them ‘^Mesozoic sandstones and shales.^’ 
G. Jjaterite. 
As the laterite has been studied maiidy iir the field, it is best to discuss 
it at length now. ITiere are two distinct types: liigh- and low-level laterite. 
1 . Jligli-LereJ Laterite. 
The edge of this capping lies at a height of belAveen 709 and 850 feet 
aboA'e sea level. Although in places it is marked by a ‘‘breakaway” — a 
scarp (up to 30 feet high) top]jed by a few feet of solid pisolitic laterite, — 
more often the edge of the latru’ite is covered by a. feAV feet of sand and 
rubble so that it cannot l)e n]ai)ped with certainty. Furthermore, laterite 
is found in places at a level below that of the breaka.Avay only a short 
distanc(* away. North cd' stream A, there is a small out('rop, .30 feet below 
the scaiq), and north of Wright Brook, a ridge; about 50 feed, below the 
breakaAvay, projects for Vi mile. OTiese outcro])s are separatee! from the* 
main plateau by rubble which in olhew places extends for 200 fe^et bedow 
the breaka\Amy. Detritus obscures most granitic outci'ops over the whole 
scarp east of the SeHfe)rth Home, but it is now being cut thi'ough by a stream. 
A pit has been elug a chain away from and a few feet above the edge 
of the laterite outcrop, ea^t ol* tlie White Hock Quarry. It- passes through 
about 30 feet of laterite, underlain by pink and white clay. Laterite must 
therefore extend below the level of the breakaAvay, and, as has just been 
mentioned, its boundary is likely to be obscure, especially where there is no 
breakaway. 
As the laterite capping formed (Woolnougli, 1918) on the surface 
of the peneplain, differences of laterite level are due to initial relief of thd 
peneplain. The thickne.ss of the capping probably rarely exceeds the 30 
feet found in the pit (above). In places, it is much thinner, for epidiorite 
fragments have been found well above the base of the laterite. 
