18 HORN EXPEDITION — PHYSICAL GEOGRAPHY. 



Besides tlie great quantity of water directly evaporated, some of it goes to 

 supply the wants of the Gums which line the banks of the creeks. In times of 

 heavy flood some of the water may reach Lake Eyre along the channel of the 

 Macumba, while a large portion filters gradually down towards Lake Eyre through 

 the bed of the creek at a moderate depth from the sui-face. Lastly, an important 

 portion of the rainfall percolates through the outcrops of the porous strata of 

 Cretaceous age, travels downwards towards Lake Eyre, and forms the supply from 

 which tlie artesian water is derived. Some of this water reaches the surface again 

 in several localities by natural outlets, and issues from the mouths of mound 

 springs. Some of the water probably percolates thi'ough the sand and gravel of 

 the river beds, and in this way reaches the water-bearing strata of the artesian 

 basin. 



C— GORGES AND GAPS. 



The next features to be described ai-e 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 200 to 700 or 800 feet above the valleys. Their length varies exceedingly, 

 while in widtli they range from a few feet to forty or flfty yards. Many of the 

 rivers after flowing equatorially for some distance along the longitudinal valleys 

 turn abruptly to the south, and cross the ranges thi'ough these gaps. 



In most of the gaps ai^e beautiful pools of clear fresh water, in which large 

 numbers of fish live, belonging, however, to a few species only. It has been 

 thought til at these gaps occupy the sites of faults in the strata, but it appears to 

 us to be (juite 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 liave 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, i.e., the portion which has an internal 

 drainage, comprises an area of nearly a million aiad a half squai-e miles, and is, 



