72 
has been eroded out of an anticlinal arch, while the rocks of the corresponding 
synclinal trough now form the George Gill and Levi Ranges. The influence of 
(2), as might have been expected, is to be observed throughout this region, the 
greater number of the valleys within these ranges having been, to a great extent, 
eroded out of the limestone beds. 
The angle of inclination of the mountain slopes depends to a great extent on 
the nature of the strata and on their dip. On one side of the ridge the slope 
often conforms to the dip of the strata when these are inclined at fairly steep 
angles. On the south side of the quartzite ridge, for instance, which forms the 
northern boundary of this area, the inclination of the strata (quartzite and 
limestone), as well as that of the mountain slope, is from 60° to 70°. 
The north face of this ridge, however, presents for almost the total thickness 
of the quartzite a sheer perpendicular escarpment extending from the summit 
downwards 200ft. or 300ft. Below this quartzite, as at Mount Gillen, the 
remaining 600ft. to 700ft. of this face, composed of Pre-Cambrian gneiss, has 
a slope not exceeding 30° to 40°. A similar difference in the inclination of the 
slopes of a ridge is also to be seen in the case of the Mereenie Escarpment, 
which probably extends almost continuously as far east as the Finke River. 
On the north side there is a steep, almost perpendicular, escarpment from 
500ft. to 600ft. high, whereas on the south the slope conforms to the dip of the 
strata. 
In those ranges where the sandstone is dipping at very l»w angles, as in the 
George Gill and Levi Ranges, we find very steep, almost vertical, escarpments 
on both sides. The northern face of Levi Range, for ins’ance, rises to an 
elevation of about 500ft. above the valley of Petermann Creek, for nearly 300ft. 
of which it is almost vertical. At the foot of this escarpment there is a talus 
slope nearly 200ft. high, with an inclination of about 3v°. 
(3) Creraceous TaBLE-TtopreD Hitis AND ‘TABLELANDS. 
Although there are no mountains or mountain ranges worthy of the name 
within the Cretaceous division, the altitude of the Cretaceous plains near the 
northern limit of this area is as much as 1,000ft. above sea level. These elevated 
plains slope gradually towards Lake Eyre from an altitude of 1,000ft. above sea 
level to 39ft. below sea level at Lake Eyre. : 
Rising out of these plains are numerous table-topped hills and low flat ranges 
and tablelands, isolated from one another by denudation. Some of the highest 
of these rise to an altitude of from 300ft. to 400ft. above the surrounding plains, 
and thus in the northern part of this Cretaceous area to 1,2U0ft. to 1,300ft. 
above sea level. These isolated masses are separated by ‘‘ stony’ and ‘‘loamy ”’ 
plains and ‘‘sandhills.”’ The hills are usually crowned by a layer a few feet 
in thickness of an exceedingly hard rock, representing sometimes a sandstone, 
sometimes a grit, and at other times a finer-grained and more argillaceous rock. 
Between the grains of this rock hydrated silica has been deposited from solution. 
To the presence of this cement is to be attributed its extreme hardness and often 
more or less conchoidal fracture. ‘This so-called ‘‘ Desert Sandstone,’ or 
porcellanite, when finer-grained and more argillaceous, protects the underlying 
strata fiom denudation in a way that may be compared to the protective action 
of the boulders in the case of the famous earth-pillars of certain villages in the 
Tyrol. 
