51 



period subsequent to the deposition of the terraced formation,, 

 and that a sectional line from the Lake Eyre basin to the top of 

 the plateau is through a regular sequence of formations in des- 

 cending order, with only the one interruption caused by the ex- 

 posure of schist and granite on the Finke at Polly's Springs. 

 That this schist-ridge was upheaved long antecedent to the 

 deposition of the strata now surrounding it is certain, otherwise 

 the beds would have been tilted and the terraced formation of 

 calcareous strata l^rought to the surface. It is probably an out- 

 lying extension of tlie rocks forming the base of the Musgrave 

 Ranges. The synclinal undulations of the terraces becoming 

 more and more sharply curved in approaching the Macdonnells, 

 and finally becoming faulted and dislocated along with the rocks 

 forming those ranges, is proof that tlieir folding is due to the 

 upheaving of that mass, and therefore that this portion of the 

 plateau has been upheaved since the deposition of the terraced 

 formation. From Dalhousie Springs to the Everard Plain on 

 the top of the plateau is 300 miles in a straight line, and from 

 Lake Eyre the distance to the same point is 500 miles. By the 

 beds of the watercourses which drain its surface, this section is 

 one continued gentle slope passing from sea level up to 2,700 

 feet, and so uniform is it that there is not a waterfall, or 

 even a rapid, throughout the whole extent of the main creek 

 channels. With regard to the geological age of this region 

 the presence of fossils on the face of the slope up to an 

 altitude of 800 feet places us on pretty sure ground and 

 shows the age of deposition to be Mesozoic (and probably 

 Cretaceous). Farther up the slope there must exist doubt 

 until fossils are actually discovered, though it is a reason- 

 able presumption that the beds throughout belong to one great 

 series. With regard to the Macdonnells and the jDlateau the 

 conformability of the rocks is certain, but should their age be 

 considered archiean solely on the ground of their high degree of 

 metamorphism ? The diorite eruptions have evidently been the 

 chief cause of metamorphism, and to their general strike or direc- 

 tion the whole northern side of the plateau conforms. 



Further, did this intrusion of diorite take place late in the 

 Mesozoic period, and uplift Central Australia into dry land. In 

 such a case the great and gently sloping swell would eventually be 

 cut up by surface waters and wind. The continental climate 

 thus engendered being similar to that now existing would seldom 

 cause floods to deepen the watercourses, and we might rather ex- 

 pect short creeks which soon spread out over the surface and 

 their waters thus completely evaporate, leaving behind the 

 matters previously held in solution. The flnest particles would 

 be carried furthest by the running waters, and two classes of 



