PRE-CAMBRIAN AREAS. 189 



altered doleritic intrusions are found in both the newer and the 

 older series. 



The Cambrian basin comprises an immensely thick series of 

 breccias, conglomerates, and true glacial tillite ; with these are 

 associated repeatedly recurring grits, sandstones, slates and cal- 

 careous beds. A single granite boulder in the tillite outcrop 

 measures 22 feet in length. Overlying the pre-Cambrian granite 

 at the Boolcoomatta head station is a granite breccia several 

 hundred feet in thickness. This breccia is composed of fragments 

 of the underlying granite. As is the case in the region near 

 Adelaide, ilmenite grains are abundant in these basal Cambrian 

 beds, in some places becoming so concentrated as to form bands of 

 almost pure ilmenite rock. 



The second section (Plate 11) refers to an area lying west of 

 Lake Frome, near the mining district of Yudnamutana. Here I 

 have determined a belt of pre-Cambrian rocks surrounded by 

 steeply dipping and undulating Cambrian sediments. The section 

 is a semi-diagrammatic representation intended to make clear the 

 salient geological features. 



The pre-Cambrian rock types are here more varied than at 

 Olary. Granite, gneiss and schist predominate. Notably in this 

 locality there is an effusive equivalent of the granite. This is a 

 striking quartz porphyry, in which the quartz idiomorphs are of a 

 iaint bluish tinge. Another allied rock is a felspar porphyry with 

 porphyritic pink orthoclases. Amongst these igneous rocks are 

 bands varying from a few inches to several hundred yards in width 

 of micaceous schists, owing their origin, apparently, to pneuma- 

 tolitic alteration of portion of the igneous rock itself. These schists 

 may be composed either exclusively of mica, or more usually mica 

 accompanied by a greater or less proportion of blue or colourless 

 corundum, pleonaste, or magnetite. Large nodules of black tour- 

 maline are often met with, and when this is the case there is 

 frequently a noticeable proportion of yellow monazite. Veins of 

 coarse white apatite are also known in the mica belts. At the 

 largest of these outcrops, that between Mt. Pitts and Mt. Painter, a 

 transition to a very tough cordierite, sillimanite, corundum schist 

 takes place. Several miles south-east of Mt. Painter the schists are 

 more usually types suggestive of the metamorphism of sedimentary 

 Tocks. The sedimentary origin is quite obvious in the case of a 

 very thick quartzite series met with in the neighbourhood of the 

 Tock hole on Arkaroola Creek, where the latter takes a sudden turn 

 across the strike of the beds. This quartzite is light grey in colour, 

 fine-grained and compact. The bedding is distinctly marked by 

 fine dark lines caused by concentration of ilmenite grains along the 

 bedding planes. Beautifully ripple-marked surfaces are frequently 

 ■exposed on the cliff faces. 



Intrusive basic igneous rocks are of common occurrence in this 

 pre-Cambrian series. These are partly coarse and fine grained 



