103 



Happy Valley level, is best explained as a fault-block, which 

 has slipped off from the western face of the range in the 

 settlement following on its elevation, or possibly may never 

 have been elevated to the same height as the main peneplain. 

 Such areas are bounded on two sides by sharp fault-scarps 

 (e.g., Tapley Hill, Mitcham Hill, or Anstey Hill), and have 

 a general inclination towards the south. 



Geology of Houghton District. 



The scheme of the geological history of the Mount Lofty 

 Ranges has been so clearly demonstrated by Mr. Howchin that 

 it will suffice to state very briefly the nature of the occur- 

 rences in this area. 



The Pre-Camhrian ('') rocks consist of schists and an in- 

 trusive plutonic series. The schists are sedimentary, mainly 

 quartzose mica schists, often with considerable felspar and 

 sericite. In one place it is strongly calcareous, while included 

 in the plutonic mass is a lens-shaped area of hsematite schist 

 rather titaniferous. It might be here remarked that the pre- 

 sence of titanium is quite a marked feature, even of the mica 

 ■schists. The boundary between these and the Cambrian-beds 

 follows the Torrens down to Deep Creek, and then strikes 

 north up that gully past Highercombe, through Houghton, 

 and thence along the Little Para, and keeping in the same 

 direction beyond the bend in that river. To the east they 

 •extend into the Hundred of Para Wirra. 



These rocks are intruded by a series of plutonic rocks 

 which, on the field classification adopted by Salisbury and 

 Chamberlain, or by the authors of 'The Quantitative Classi- 

 fication of Igneous Rocks," would be classed as syenites on 

 account of the predominance of felspar. Microscopical in- 

 vestigation, however, shows that in the majority of cases the 

 predominant felspar is an acid plagioclase, so that the rock 

 is, strictly speaking, a diorite. In some instances orthoclase 

 or microcline is in excess, and the rock is a syenite. The 

 ferromagnesian silicate is a uralitized diopside. The most 

 noticeable mineralogical feature is the abundance of ilmenite. 

 Epidote is a common secondary mineral. In grainsize the 

 rocks vary considerably. Generally they are fine-grained 

 (diameter of grain 1 mm., and at times much finer), 

 while on the other hand they sometimes reach about 

 5 mm. in coarseness. Usually the rock is distinctly 

 banded, the lighter and darker minerals segregating 

 into parallel layers which have an easterly dip at 

 varying angles. With these rocks there often occur 



(4) See W. Howchin, Trans. Roy. Soc, S.A., 1906, p. 256. 



