384 PHYSICAL STEIJCTUEE AXD GEOLOGY OF AUSTRALIA, 



This is the proper place to mention that the surface and some 

 small portion of the upper part of gold veins are usually found 

 to be of much greater richness than the lower parts ; and this is 

 not because gold favours one portion of a vein more than another, 

 but because the surface represents the gradual detrital accumula- 

 tion from slow weathering in the course of ages. 



The drift or sub-aerial deposits belonging to recent geological 

 periods are in Australia very important and extensive. An 

 illustration from one portion of Australia will explain the whole. 

 In the Murray basin, and through much of South Australia on 

 the west side of Spencer's Grulf, and north-west of Port Lincoln, 

 there are many isolated granite hills, which are the outcrops of 

 the basis of the whole continent. All around them is a sandy 

 desert supporting low shrubby thickets or " scrubs " of Acacia, 

 Eucalyptus, Cryptandra, Melaleuca, &c. The sand is a mass of 

 rounded grains of true desert character, that is of grains rounded 

 by having been blown about. It frequently lies in ridges and 

 hills like the waves of the sea, but generally covered with vege- 

 tation. The hills are interrupted by yellow clay flats, with an 

 open forest and a soil which a very little moisture renders boggy. 

 Both these accumulations arise from the slow subaerial weather- 

 ing of the granite. The quartz grains resist decomposition, and 

 get carried about by the wind. The felspar and mica decompose 

 rapidly, and form the basis of the clays to which the small portion 

 of iron per-oxides gives the yellow colour. It is remarked that 

 wells sunk into this clay only furnish an almost undrinkable 

 brackish water, doubtless from the salts of soda, potash, and iron 

 which the felspars contain. An analysis of these waters shows 

 them to contain such salts as chloride of sodium, sulphates of 

 soda, lime, magnesia, and potash, with variable proportions of 

 silica and iron. All these can be referred to chemical decompo- 

 sition from granites, notably felspars and mica. The taste of 

 salt (sodium chloride) by no means predominates in them. TIio 

 general surface of this kind of country, with the exception of the 



