HORN EXPEDITION — PHYSICAL GEOORAPHV. • 17 



percolation of the water Ijelow the surface. When the Hood-waters abate over these 

 portions, tlier(! usually remain a string of waterholes occupying tlie depressions in 

 the river bed. If rain does not fall for some time these one after another lose 

 their water by evaporation and become dry. Only those which occupy the deepest 

 and most sheltered depressions hold out for any length of time. Such waterholes 

 prevail in the Cretaceous area as in the Stevenson, Maciunba and Coglin llivei's, 

 and form the chief sources of supply in the districts where they occur. 



What a valuable provision of nature in reality are the sandy beds of the 

 rivers ! Through the sand and gravel the water creeps slowly down its course 

 protected to a great extent from evaporation, and here it may generally be obtained 

 by sinking ; whereas if it had remained on the surface it would hav(^ been I'apidly 

 evaporated. Only in those places where ledges of rock cross the channel do the 

 waters appear at the surface, and then usually for a short distance only, disappear- 

 ing again in the sand on the other side of the rocky bars. 



Seldom do the flood-watei's of the Finke and Macumba flow over the surface 

 to Lake Eyre; for the lacustrine delta of these rivers, consisting for tlie greater 

 part of deposits of sand and loam which have l)een accumulating during the Late- 

 Tertiary and Post-Tertiary epochs absorb the immense body of water- brouglit 

 down by these rivers. 



(/) Rainfall in its Relation to Surface Water. — The rainfall thioughout the 

 area of the Finke Basin is somewhat variable, ranging from an average cjf less 

 than five inches per year in the central and southern portions to ten or twelve 

 inches over much of the mountainous country in the northern part of the basin. 

 The mean annual rainfall throughout the basin cannot be more than six to seven 

 inches. 



A great part of the moisture that falls as rain thi'oughout this area is lost by 



evaporation. From claypans and all shallow depressions which expose large 



surfaces to the desiccating agents, the water disappears in two or three months 



after a fall of rain. From the waterholes, too, during the dry season the water 



disappears at an alarming rate. Waterholes which have been examined by one 



explorer during a good season, and declared to be permanent, have, when examined 



later by a second explorer in a rathei- dry season, been often found to be dry or 



nearly so. Exploiters have indeed been rather too hasty in forming conclusions as 



to the permanency of waterholes, which, visited perhaps during a good season, 



contained a good supply of water, but which had in reality no element of 



permanency. Tn the great majority of cases they are not fed by springs noi' 



situated in places sutticiently sheltered to give practical permanency t(j them. 



:i 



