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the basin than at the margins, and thereby brought to the 

 surface and exposed to view the whole section of the Cambrian 

 deposits at its greatest thickness. 



This view of the tectonic phenomena may explain a 

 few of the facts which seem to fall naturally into line with 

 it: — 



(a) The Mount Lofty beds, as constituting the lowest 

 members of a thick series which reached the point of maxi- 

 mum depression, must have been subjected to a higher degree 

 of metamorphism than the beds which occupied a shallower 

 position around the margins. This is borne out by the 

 geological features, as there is a gradual increase of meta- 

 morphic intensity as the beds are followed in a downward 

 order towards the Cambrian base. The great field of igneous 

 intrusion, which was developed on the eastern side of the 

 basin, greatly modified the petrographic features on that side 

 and had the effect of obscuring the succession. 



The Yorke Peninsula region was probably situated on 

 the western edge of the Cambrian basin, or otherwise occupied 

 a high-level position as compared with the deeper portion, 

 and therefore the isogeotherm conditions would be in strong 

 contrast to those under which the more deep-seated beds were 

 laid down. In harmony with this view the basal grits and 

 conglomerates are but feebly fortified by secondary silici- 

 fication — even less so than the comparatively recent alluvial 

 grits of the same locality — and they give no evidence of 

 powerful shearing, rock flowage, or mylonitic effects, such as 

 are seen in the Mount Lofty Ranges, as, for example, at the 

 Grey Spur in the Inman Valley. The metamorphism on 

 Yorke Peninsula has been of the mildest type; even quartz 

 veins in these beds are rarities, and the limestones have only 

 undergone such slight molecular reconstructions that might 

 take place in beds of this type at only moderate depths. 



fb) Whilst on the eastern side of Gulf St. Vincent the 

 Cambrian beds are greatly disturbed and thrown into acute 

 folds and overthrusts (in which the beds of the Upper Cam- 

 brian have shared as well as those of the Lower), the beds 

 on Yorke Peninsula give only slight evidences of orogenic 

 activity. The beds, if not horizontal, make long undulations 

 within the range of a few degrees of dip, the highest inclin- 

 ation being 40°, and this occurs on the beach at their eastern 

 limits and on the margin of the rift valley. This slight 

 deformation is what might be expected under the circum- 

 stances. The Cambrian beds formed a relatively thin cap- 

 ping on the strong foreland which not only presented a 

 barrier to the oncoming earth-waves, but at the same time 

 protected the overlying Cambrian sediments from the thrusts 



