4 HORN EXPEDITION — PHYSICAL GEOGRAPHY. 



Tlie original stratification of the once sedimentary strata and the joints, that 

 were perhaps present in the original granites, have played to all appearances no 

 pait in the moulding of the present physical features out of the metamorphic rocks, 

 the age of which is considered to be Pre-Canibrian. Rising abruptly out of the 

 elevated area of the McDonnell Ranges, with no linear arrangement, but irregu- 

 larly distributed, are a sei-ies of eminences, whose summits are in some cases, as in 

 that of Belt Range and Mount Sender, capped by a northern extension of Ordovician 

 quartzite. To the presence of this protective covering, which has effectually 

 warded off' the levelling forces of nature from the underlying less weather-resisting 

 metamorphic rocks, is probably due the comparative elevation of these peaks. 



Ranged in order west to east the altitudes above sea level of the chief peaks 

 are as follows : — Mount Edward in Belt Range, 4649 feet ; Mount Heughlin, 

 4756 feet ; Mount Zeil, 4040 feet ; Mount Sender, 4496 feet ; and Mount Giles, 

 4210 feet. 



The average elevation of the surrounding country is over 2000 feet above sea 

 level, so that these mountains are not so prominent as one might be inclined to 

 imagine judging alone from their altitude above sea level. They are nearly all 

 accessible, as the slopes are not usually very precipitous, except for example the 

 Belt Range (Fig. 3). 



The movements of the earth's crust, to which these ranges bear witness, were 

 to a gi'eat extent of Pre-Ordovician age, and were continued, though much 

 diminished in intensity, down to Post-Cretaceous times. During the earlier part 

 of the Ordovician period the Pre-Cambrian rocks probably underwent subsidence, 

 so as to allow of the deposition of Ordovician sediment that originally covered 

 much at least of the area now occupied by Pre-Cambrian rocks. Later they 

 partook of the Post-Ordoviciaii ujjlieaval, which converted much of the area 

 occupied by Ordovician sea into dry land. Latei' again this area has probably 

 participated in the gentle and gi-adual Post-Cretaceous upheaval, to which are 

 attributable the very slight undulations in the Desert Sandstone. 



The extreme metamorphism of the Pre-Candjrian rocks is to a great degree 

 Pre-Ordovician ; but, as indicated by the gneissic character of much of the intrusive 

 granites, it was partly at least contemporaneous with that of the Ordovician 

 quartzites, etc. As should, perhaps, have been pointed out previously, the elevated 

 area occupied by the McDonnell Ranges forms a great part of the northern 

 boundary of the Lake Eyre i^asin. In travelling soutliwards from this elevated 

 region towarils the centre of the basin, we descend by means of a series of terraces 



