MESOZOIC PLAINS OF SOUTH AUSTRALIA. 241 



The probable explanation of these facts appears to be that the 

 greater part of the laterite is an altered basalt tuft' belonging to 

 the earlier basalt eruptions of the Eocene Period, or possibly to a 

 time intermediate between that of the older and newer basalts. 

 It is highly probable that some of the laterite is an altered 

 scoriaceous basalt. Tliat water has played an important part in 

 altering the tufts and scorise, and possibly in re-distributing them 

 is proved by the occurrence, though exceptional, of small fragments 

 of plants intermixed with the laterite, and by the lenticular beds 

 of limonite occupying local depressions on its surface. 



5.— THE MESOZOIC PLAINS OF SOUTH AUSTRALIA, 

 (South of Lat. 26°). 



By H. Y. Lyell Brown, F.G.S., Government Geologist, South 



Australia. 



The area over which the Mesozoic beds form the surface rock, or 

 are only thinly covered by those of Tertiary formation, extends 

 east from the boundaries of New South Wales and Queensland, 

 westward to the boundary of Western Australia, northward it is 

 bounded by the Musgrave Ranges and Latitude 26°, and southward 

 by an irregular line extending fi'om the northern edge of the 

 Nullarbor Plains in Western Australia, round by the Warburton 

 Range, and the northern extremity of the main range to the 

 vicinity of Lake Frome, near which it passes into New South 

 Wales. Here and there along the telegraph line, from Hergott 

 Springs to the Peak and Charlotte Waters, the Dennison, Margaret 

 and other ranges, composed of Primary rocks, pi'otrude through it 

 in the form of islands, rising above the general surface in places 

 to a height of 1,000 feet or more. 



Lakes Eyre, Blanche, and Frome, are the principal lakes of this 

 region, although there are numerous others dotted over the area. 

 The largest rivers are the Diamentina and the Cooper, which enter 

 the north-east corner of the colony from Queensland, and spread 

 out into enormous lakes, connected by creeks and watercourses, 

 by which, in times of flood, the lakes are filled. Other rivers are 

 the Treuer or Macumba, and the Neales, with their numerous 

 tributaries, rising in tlie neighbourhood of the Musgrave Ranges, 

 and flowing eastward into Lake Eyre, with the Warburton, Clayton, 

 Frome, and many other smaller creeks. The general level of this 

 vast territory varies from some two hundred feet above sea 

 level to twenty-five or thirty feet below it, in the case of Lake 

 Q 



