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C.—GORGES AND GAPS. 
The next features to be described are the gorges and gaps, which are rather 
numerous in the ranges in the northern portion of the region under consideration, 
and through which many of the rivers flow on their ways southwards towards 
Lake Eyre. These features are in the form of narrow rocky passes with walls 
usually of quartzite, but sometimes of sandstone, rising almost vertically to 
heights varying from 200ft. to 700ft. or 800ft. above the valleys. Their length 
varies exceedingly, while in width they range from a few feet to 40yds. or , 
50yds. Many of the rivers after flowing equatorially for some distance along 
the longitudinal valleys turn abruptly to the south, and cross the ranges through 
these gaps. 
In most of the gaps are beautiful pools of clear fresh water, in which large 
numbers of fish live, belonging, however, to a few species only. It has been 
thought that these gaps occupy the sites of faults in the strata, but it appears 
to us to be quite unnecessary to call in the aid of such dislocations to account 
for their origin. The origin of the majority of the gaps is probably due to the 
erosion of the river beds in the positions of the present gaps keeping pace with 
the upheaval and folding of the strata in those places. By thus lowering their 
channels the rivers have maintained their original positions. In a few cases, 
however, the gorges may owe their existence directly to faults, the water readily 
eroding a passage for itself along the fault planes and their associated cracks. 
D.—LAKES. 
The interior district of Australia, z.e., the portion which has an internal 
drainage, comprises an area of nearly a million and a half square miles, and is, 
therefore, seven times as large as the Great Basin of North America. It will 
thus be readily understood that the lakes, which form the centres of the several 
drainage basins, are physical features of the greatest interest and importance. 
Surrounding the area of internal drainage is a strip of country stretching inland 
from the coast for varying distances, throughout which the rivers carry the 
surface water to the ocean. 
Only two lakes occur in the region considered in this paper, viz., Lakes Eyre 
and Amadeus, each of which forms the centre of one of the divisions of the 
internal basin. Both of these are fast passing into the state of dry basins. 
This is due in the first place to the aridity of the climate throughout the region 
occupied by them, from which results an almost total absence of superficial flow 
of water into the lakes, especially Lake Amadeus ; and in the second place it is 
due to the accumulation in the lakes of sand, &c., transported thither by running 
water or wind. These statements apply in a special manner to Lake Amadeus, 
the absence of surface water near its western extremity at any rate being 
specially noticed by Mr. Tietkens. 
From the sandhills bordering the lake near Gosse’s Crossing, 7.¢., towards its 
eastern extremity, we could make out no water on its surface. There the dry 
bed of the lake was crossed without trouble en rowte to Ayers Rock and Mount 
Olga. 
Of these two lakes Eyre is the larger and the more important, and will be 
considered first. 
F 
