1120 The Geology of Central Australia. [November 
feet; and the eastern cordillera 3500 or 4000 feet. Thus there is 
a gradual slope from the east and the west toward Lake Eyre. 
The Peake range, arising as it does, roughly divides the great 
basin into an eastern and western portion. It is worthy of remark 
that the Peake range is flanked by the Jurassic formation on each 
side, while the Cretaceous beds flank the ranges on the extreme 
eastern and western boundaries. There is evidence to show, 
however, that the latter beds are underlaid by Jurassic rocks. 
The Musgrave range consists of a central core of granite, flanked 
on either side by a great variety of gneissoid rocks much folded 
and inclined at a high angle. Many dykes of doleritic rocks cut 
through the metamorphic gneisses. 
The granite varies greatly in character within short distances. 
The most common is a large porphyritic variety with a coarsely 
granular base. The orthoclase crystals often measure three 
inches in length. Mineralogically the granite is usually of the 
normal ternary kind, ¢. e., consists of orthoclase, quartz and mica. 
The mica is almost iivasiebly biotite. Sometimes oligoclase is 
found with the orthoclase, and often completely replaces it. In 
other localities the granite loses its quartz and mica, and those 
minerals are replaced by hornblende, the rock becoming a pur 
syenite. Then again in the syenitic form the orthoclase pok 
subordinate place and is replaced by oligoclase. In this variety 
have found small quantities of augite as an accessory n 
These varieties of composition all occur in the same mass na 
may be stated as follows: 
a, sheen —Consists of | ai quartz and biotite. Tourmaline often present 
n small pockets or n 
ó. Waiti ndic.—Same as aber but contains hornblende. 
c. Oligoclase-bearing. —Oligoclase present and sometimes completely 
thoclase 
d. SyeniticHomblen de replacing the biotite. Quartz te? or nearly pepo gy. 
e. do. Oligoclase-bearing. —Consists of hornblende and oligo sa as 
Resembles the rock called trachydolerite (Abich). Augite aig pond | 
an accessory mineral. i 
The granite may thus be said to vary from its normal pe : 
sition to a syenite. The normal rock is by far the most abu 
Next in order is the syenitic variety. 
The gneissoid rocks flanking the granite vary 
position and physical characters as the granite itse 
_ common kind is a light-gray coarse granular gneiss C : 
quartz, orthoclase and muscovite or biotite, sometimes ] 
replacing the 
as much ia oo 
if. The most 
onsisting of - 
