HORN EXPEDITION — PHYSICAL GEOGRAPHY. 



A.— MOUNTAINS. 



In the consideration of the physical geograpliy of this country the mountain 

 system should first demand our attention, as being of primary importance, for on 

 it not only does the trend of the main valleys depend but also the size, number, 

 and even the very existence of the rivers, by reason of its influence on the wind 

 and rain. For the above reasons the McDonnell Ranges form the most important 

 physiographic feature of Central Australia. Were it not for the presence of this 

 chain of comparatively elevated land, with its important influence on the 

 meteorology of the surrounding country, the greater part of the interior would 

 re.semble a sterile desert, which is the actual condition of portions of the stony and 

 sandy plains after a more than usually prolonged drought. 



The most important mountain i-ange within the area under consideration is 

 doubtless that of the McDonnell, which represents the much -denuded crest of one 

 of the highest folds, into which the crust of the earth within this area lias been 

 thrown. In the crumpling oi tlie earth's crust the highest anticlines, being on the 

 lines of maximum disturbance, usually have cores of plutonic rocks. Tliis seems 

 to have been the case with a great portion of the McDonnell Ranges, subsequent 

 movements having metamorphosed the originally plutonic rocks, thus causing 

 doubt as to their eruptive origin. 



The mountain system of Central Australia does not consist alone of the 

 McDonnell Ranges and tliose immediately associated with them to the north and 

 north-east, as the Hart and Strangways Ranges, but includes also a number of 

 parallel ranges lying to the south, such as the James, Waterhouse, George Gill, 

 Levi, and Chandler Ranges, all representing the arches or troughs of the folds 

 produced by earth movements in past geological time. Examples of ranges occupy- 

 ing the troughs of the earth-folds are to be found in the case of the George Gill 

 and Levi Range. In each of these ranges, which really are portions of one and 

 the same range (their continuity being broken merely by the transverse valley of 

 Trickett Creek, a trilmtary of Petermann Creek, the latter a branch of the 

 Palmer), the rocks occupy a perfect synclinal trough, in which the sandstone dips 

 from the north and south towards the centre of the range at an angle of from 

 10° to 20° 



The mountain system of Central Austi-alia may be conveniently treated in 

 three divisions, the mountain ranges in each division for the most pai't comprising 

 rocks of one and the same geological age, while they differ from those of the other 

 divisions. 



