74 
The Mount Davis chain includes, among others, a large 
intrusion of granular olivine-gabbro, "^ varying in colour 
from dirty green, through various shades of green, to faint 
blue. In the last case the predominance of plagioclase fel- 
spar and the presence of only a small amount of olivine have 
produced the bluish tint. The intrusion trends east and 
west as a massive, rugged chain, flanked by less conspicuous 
diorite dykes. 
The latter, though individually smaller, are very nume- 
rous. Their direction of intrusion possesses no regularity, 
often cutting one another at various angles. Upon one hill, 
about three miles south-east of Mount Davis, two conspicuous 
diorite dykes can be traced up the hill slope. These dykes 
gradually converge towards the summit of the hill, where 
they ultimately cross one another at an angle of about 30°, 
each continuing its own course after the point of crossing. 
The direction of intrusion of the diorite appears more con- 
stant (east and west) on the northern side of ih^ ranges 
than is the case of the more numerous examples on the 
south. 
Very often smaller dykes can be traced in a direction 
nearly at right angles to the larger, from which latter they 
have been injected into minor fissures of the rock. The 
trend of these smaller dykes, in several cases, was noticed 
to correspond with that of the planes of foliation of the in- 
truded gneiss, and their outcrops can be traced down to the 
adjacent sandy flats, from which they stand out, by their 
superior weathering, as marked, low, parallel walls, f As a 
general rule the diorite rock of the Tomkinson Ranges is of 
one type only : a finely crystalline, black-looking (horn- 
blendic) variety. 
A few miles south of Mount Davis a slight exposure of 
graphic granite occurs. The quartz that produces the hiero- 
glyphic markings on the surface of the rock is colourless and 
embedded in a red orthoclase felspar matrix. The whole 
rock is traversed by veinlets of crystalline epidote. 
* J. Carruthers, op. cit.: 'The Tomkinson Ranges . . . 
are composed of grey and red granite, with large outcrops or dykes 
of basalt." No basalt was found in the neighbourliood of the 
Tomkinson Ranges, and it is possible that the gabbro was mis- 
taken for basalt by Carruthers. "W. C. Gosse, Report and Diary 
of Central and Western Exploring Expedition, 1873, Parliamen- 
tary Paper No. 48, House Assembly (1874). p«ge 13, writes: — 
"Mount Davis must be at least 1,.5()0 ft. high. This portion of 
the range is composed chiefly of grey granite." W. R. Murray, 
Extracts Jounials of Explorations by R. T. Maurice (bv autho- 
rity : 1904, page 17). 
t Which Mr. Streich compares with the "ruined walls of 
houses." Scient. Res. Elder Expl. Exp., Tr«ns. Roy. Soc, S.A., 
vol. xvi., page 93. 
