NOTES ON A "FAYALITE BASALT" FROM 

 ONE TREE POINT. 



By 0. E. White and W. A. Macleod, B.Sc. B.A. 



The rock on which these notes have been written 

 occurs as a basaltic flow in a small promontory, on the far 

 side from Hobart, of Sandy Bay. 



The geological age of basaltic flows and tuffs is given 

 by Mr. R. M. Johnston (Systematic Account of the Geology 

 of Tasmania, p. 249 et seq.) as being younger than the 

 tertiary leaf -beds, and from the occurrence of this rock 

 and its associates, sections of which can clearly be seen on 

 the Brown's River Road, there seems to be no reason for 

 doubting that the age ascribed is approximately correct. 

 The average thickness of the flow is on the exposed section 

 about nine feet, and the extent of the sheet, as far as it 

 can be traced, is not great. {Ihid.^ p. 281.) 



This paper has been written rather to give an account of 

 the peculiar Petrographical character of this rock than to 

 attempt a description of its geological occurrence, and 

 further, such an attempt would only be a trespass on 

 ground already well worked. 



In the title the name " Fayalite Basalt " has been used 

 as descriptive of this rock. This term may be open to 

 some criticism when we consider the definitions given by 

 the leading authorities of the term " Basalt." For 

 example, Rosenbusch defines basalt as a rock consisting 

 essentially of olivine, augite, and plagioclase, and regards 

 such rocks as the tertiary and recent equivalents of olivine, 

 diabase, and melaphyre. This rock will answer the 

 requirements as to geological age, but not those relating to 

 mineralogical constituents, for in some seven sections 

 examined not a trace of augite was discovered. Olivine 

 exists as the red variety Fayalite (FeO, SiOg), and plagio- 

 clase felspar, probably as labradorite. This peculiar 

 mineralogical composition involves almost a total absence 

 of magnesia, and this absence is confirmed by chemical 

 analysis, the result of which is given below. From a 

 structural point of view the term ** Fayalite Basalt," seems 



