PRESIDENTIAL ADDRESS. 21 



■earlier Mesozoic rocks. In New South Wales they occur as far 

 south as Bidura, near Balranald, and they have been recognised 

 in many other places outside the limits of the Artesian basin. 

 They have, however, suffered to an enormous extent from denuda- 

 tion, and are generally seen as small hills, forming isolated outliers 

 of what was once a continuous far-reaching deposit, 



"The desert sandstones are well represented in the neighbour r 

 hood of Mount Oxley, Milparinka, Mount Poole, Mount Stuart, 

 and the Gray ranges to the north of the latter. There are two 

 typical varieties of rock to be seen in these localities. One is a 

 soft grayish white sandstone, passing in places into a coarse grit, 

 while the other is an intensely hard but brittle porcelainised rock, 

 which has the appearance of having originally been a porous grit, 

 but which has been indurated by having all the interstices between 

 the sand grains filled with secondary silica, which gives it a very 

 opaline appearance when examined under a lens. One of the 

 peculiarities of this rock is the manner in which it breaks up on 

 the hill tops. The intense heat of the sun in summer raises the 

 rock to a high temperature, and when this is followed by the 

 sudden cooling effect of a thunderstorm, the large blocks of stone 

 exfoliate and break up rapidly; it is consequently somewhat rare 

 to see an outcrop of solid beds — the summits of the hills being 

 covered with a rough shingle, which is so characteristic of Sturt's 

 Stony Desert, in the neighbourhood of Milparinka and Mount 

 Poole. This rock rings like porcelain when struck, and breaks 

 with a conchoidal fracture. There seems to be very little doubt 

 that it was originally identical with the softer variety of sandstone, 

 and that the metamorphism has been caused by the action of 

 thermal springs which deposited silica between the sand grains. 



"The desert sandstone occurs in this district in isolated ranges 

 or hills, which, as a rule, have steep escarpments. The beds are 

 generally horizontal or dip at a low angle, and are traversed by 

 vertical joints. Near the top of the series there is a bed of con- 

 glomerate, a few inches thick, consisting of pebbles of an infinite 

 variety of colour, and owing to the breaking up of this conglomerate 



