40 nORN EXPEDITIOX — GENERAL GEOLOGY. 



they disappear under the quartzites and limestones of Lower Silurian age, appear- 

 ing at the surface, according to Mr. East, as a low ridge, which crosses the bed of 

 the Finke eighteen miles above Crown Point Cattle Station. Still further south 

 they probably form a great part of the Peake and Dennison Ranges ; and lastly, 

 a great part of the country to the east of Flinders Range is composed of them — 

 at all events of Pre-Cambrian rocks — being in several localities found to be over- 

 lain by strata containing Cambrian fossils. In the McDonnell Ranges alone they 

 occupy an area of at least 10,000 square miles. 



(d) Structural Features. 



Messrs. Brown and Chewings both state that the Pre-Cambrian rocks are 

 distinctly stratified, and have a dofinite and determinable dip. No evidence is 

 given in support of these statements, which must therefore be received with 

 caution. In the first place they appear to Ije conclusions from the unwarrautaljle 

 assumptions that the rocks of this region are metamorphosed sedimentary strata, 

 and that the planes, which are so strongly marked in many localities, are those 

 of bedding. 



The evidence in favour of the eruptive origin of large areas of the Pre- 

 Cambrian rocks will be gone into more fully later. In the meantime we may 

 point to the porphyiitic gneissic granite on which Alice Springs Telegraph Station 

 is situated, and which covers a large area, and to the augen gneiss with lenticular 

 "eyes" of comparatively unaltered granite embedded in a ground mass of crushed 

 granitic minerals, as being strong evidence of the eruptive origin of rocks covering 

 large areas. 



Although it may be possible and even in places probable that the planes, 

 which are so strongly developed, coincide with the original planes of stratification 

 in any large area where sedimentary rocks may have been developed, yet as a 

 general rule there can be no doubt that these planes represent foliation planes. 

 This statement is greatly strengthened by the facts of the coincidence over large 

 areas of the strike of these planes, and of their great persistency ; for they are 

 traceable not only through rock-masses, the eruptive origin of which it highly 

 probable, but also even through undoubted intrusive dykes. They are, therefore, 

 planes of foliation, of stratification-foliation — that is, of foliation corresponding 

 with the original bedding planes, it may be in places ; but elsewhere assuredly 

 tliey appear to be those of cleavage-foliation. 



The foliation and extreme degree of schistocity, and the development of planes 

 of shearing and cleavage resulting from the yielding of the rocks to the lateral 



