1130 The Geology of Central Australia. {Nove | i 
is a large enough one the water finally finds its way to the large 
lakes (principally to.Lake Eyre). These lakes, if the flood is 
very heavy, become tolerably fresh, and stay so until through 
loss of water by evaporation (none of them have outlets) the 
water again becomes salt. As evaporation proceeds the water 
slowly recedes, leaving a dazzling white incrustation, and some- 
times a layer of dead fish which have been brought down by the 
flood, and when the water became too salt have been compelled 
to succumb, 
The lakes and clay-pans accordingly, unless seen in flood time, 
are generally a painfully white plain fringed by a few sickly 
Acacias. The salts are of different kinds ; the most abundant onè 
deposited is sulphate of lime (gypsum). The general form of 
crystallization of this salt is the common monoclinic twin ot 
arrowhead crystal. Often, however, it is found in large transpa 
rent slabs and in a fibrous state. The next in importance among ; 
the salts are the carbonates of lime, magnesia and soda; t f i 
chlorides of sodium and magnesium, and the sulphates of mag- i 
nesia and soda and iron. All these are derived from the decom- ‘ 
position of the rocks forming the ranges from whence the water 
courses arise and from the denudation of the outcrops of the ; 
Mesozoic rocks which underlie the whole region. The acti 
amount of rainfall on the eastern and western slopes of the boat : 
(especially the eastern) is largely in excess of the demena 7 
the watercourses, so there must be a large portion which soa% 
into the ground and drains away along the incline ge the 
interior, where it accumulates under great pressure. we si 
andt 
the case is shown by the existence of a line of cold 4 oe, 
springs that extends north and south through the central l p 
That the water comes from a great depth is evident from th ae 
of their temperature and the mounds of travertine the 
built for themselves. The overland telegraph line nearly Š fa 
the line of outcrop of these springs. This line 1s also tHe 
crop of the crystalline schists forming the Peake range © follot 
line of springs extends in an east and west direction a 
the overlap of the Mesozoic rocks upon the Palaeozoic ast 
the Flinders range. The explanation is simple. 
ing from the north-west and north-east flows al 
clines toward the central basin until it meets wit 
schists of the Peake range. It cannot pass thro 
ha barrier it 
ugh them, 
