88 



II.— DESCRIPTIVE GEOLOGY. 



Encircled towards south by the Tertiary deposits of tiie 

 NuUahor Plains, and to the north bordered by the Azoic formation 

 of the Central Australian upheavals, an area of Mesozoic forma- 

 tion extends apparently as far as long. 123°, at least in this 

 region. It is extensively covered with drift sand and other 

 recent deposits, and no high elevations exist there, only the 

 southern precipices of the flat-topped hills being denuded from 

 surface covers. 



As far as it could be ol)served the dip of the Mesozoic strata,, 

 chiefly quartzite, was generally not exceeding 1-5 degs., and was 

 found invariably towards N.N.E. 



The height of this area varies between 1,.300 and 700 feet. In 

 the metamorphic ranges of the south, however, which seem to' 

 form a barrier towards the coast plains, the height rises to 1,450 

 feet, according to aneroid measurements, and falls after passing 

 the Eraser Range, according to Mr. Lindsay's observation, 

 gradually towards the coast. 



Recent Formatioxs. 



The sand which covers so largely the area previously referred! 

 to is the result of the disintegration of the gritty quartzites and 

 sandstones of the desert. It is generally loose and wind-drifted 

 in places, thougli it is somewhat consolidated by means of a clay- 

 cement, but only on the surface, because if the uppermost crust 

 of a thickness of one or two inches is removed, the loose sand is 

 again met underneath. 



It was here where an interesting occurrence of ccolian erosion 

 could be observed. When this uppermost crust had been broken 

 thi-ough, the wind had carried away the loose sand, leaving now 

 and then mushroom-like forms of sand, consisting of a small 

 layer of consolidated sand on the top, supported by a column-like 

 neck of less-impregnated, and therefore less-compact, sand. Some- 

 times also the cover consisted of a flat pebble (see fig. 9, pi. 

 iv.). The north-easterly part of these seolian formations was 

 more excavated than the other parts, due to the prevailing winds 

 of the interior. 



The colour of the sand is generally reddish-brown, and consists,, 

 I should say, up to 95 per cent, of quartz, the balance probably 

 being lime, as the moderate occurrence of small calcareous con- 

 cretions in the sand may prove that there is some (lime), though 

 not visibly present. 



These concretions have a cylindrical form, a concentric radiated 

 structure, and are composed of sand cemented together by cal- 

 careous matter. They represent probably infiltrations and 



