450 C. E. Tilley — Metamorphism of 



Nullarbor plains, they may be regarded as a south-easterly pro- 

 longation of the great pre-Cambrian Shield of Western Australia. 

 The platform is for the most part within a few hundred feet of sea- 

 level, and large areas of the region are covered by the products of 

 long-continued weathering of the older rocks or by siliceous deposits 

 of late Tertiary age. The best sections for geological study are those 

 exposed along the coastline, and in the hilly areas of Hutchison 

 and Warrow. 



The general discussion of the sequence of the pre-Cambrian in 

 this area will be reserved for another place, and this paper will 

 be limited to a description of certain metamorphosed sediments 

 embraced within the oldest group of rocks that have been recognized, 

 and which it is proposed to designate the Hutchison series. 



Hutchison Series. 



Only remnants of this terrain are now exposed to view, the series 

 having been broken up by the great intrusions of igneous gneisses 

 that succeeded. To this group of igneous rocks, the name — 

 Flinders series — is given, from the hundred and county in which they 

 attain a widespread and typical development. Fragments of the 

 Hutchison beds have been engulfed in the Flinders gneisses, and 

 these can be recognized at points remote from typical exposures 

 of the Hutchison series. There is abundant evidence that this 

 sedimentary terrain has been invaded and intensely metamorphosed 

 by the succeeding igneous intrusions. The best exposures of the 

 Hutchison series are developed along the coast of Sleaford Bay, and 

 in the hundred of Hutchison in the hilly district west of Turnby Bay. 

 At various intermediate points between the Sleaford Bay and 

 Hutchison areas, these rocks emerge from beneath a mantle of 

 weathered products, but no extensive unweathered outcrops have 

 as yet been recognized. In a north-easterly direction in the county 

 of Jervois, R. L. Jack has described dolomites associated with 

 schists and gneisses, and doubtless these rocks represent the same 

 series along the same line of strike. Undoubtedly the best exposures 

 of these rocks are those outcropping on the shores of Sleaford Bay, 

 near Sleaford mere. Here in a section of less than half a mile a most 

 complex series of sediments is found, penetrated by granites and 

 pegmatites of the Flinders series. 



The beds, now exposed in a highly metamorphosed condition, 

 comprise mica schists, garnet gneisses, graphite schists, graphite 

 garnet gneisses, quartzites, dolomites, and calc-magnesian silicate 

 rocks. The beds strike in a north-south direction, with a high angle 

 of dip to the west (75 degrees) to vertical. Subsidiary folding and 

 faulting have been observed. On the west they are bounded by 

 granites and gneisses of the Flinders series, while on the east the beds 

 disappear beneath calcareous sand rock and travertine, which 

 here forms the coastline till the igneous rocks of Flinders intervene, 

 near the Curta Isles, across the Bay. 



