182 G< A. Cook: Geology of Cobuvy Area. 



The rock is therefore classified : — 



Class 2 - Dosalane 



Order ") - Perfelic - Germanare 



Rang 4 - Docalcic - Hessase 



Sub-rang 3 - Presodic - Hessose 



Typical low level basalt occurs in the area between the heights of 

 125 i'eet and 14-0 feet above sea level. 



High Level Basalt. — This occurs typically at heights of about 

 L'40 feet. It is a more vesicular rock than the low level type. It is- 

 also coarser in grain, with larger and more plentiful phenocrysts, 

 and. following Harker's nomenclature, may be classed as a dolerite. 

 Microscopically it is seen to contain a larger proportion of olivine- 

 than the low level type. This olivine is generally iron stained, due- 

 probably to the porous character of the rock. Augite is present, but 

 is not nearly so ophitic as in the low level type. Another chief point 

 of difference between the two is the almost total absence of glass. 

 Ilmenite and labradorite occur similarly to the occurrence in the- 

 low level type. The chief points of difference then between the two 

 types are: — The high level type is coarser in grain, has no glass,, 

 and is richer in olivine. All'these characteristics tend to show that 

 the high level type has cooled more slowly than the low level. 



This conclusion is also helped by the field characters and relations. 

 The low level basalt frequently shows prismatic cleavage, indicating 

 rather rapid cooling. The high level type on the other hand is 

 more massive, the joints being more irregular and further apart. 

 As the first specimens of the two types were collected from localities- 

 separated by the Silurian inlier North of Pentridge, it was at 

 first thought likely that two distinct basalt flows occurred in the 

 district. Further field work, however, showed that the flows were 

 united to the West of the inlier, and further penological work 

 showed that the types grade into one another, ami that the differ- 

 ences probably arose due to different conditions of cooling rather 

 than to different composition of magma. Specimens collected from 

 heights intermediate between 140 and 240 feet shew characters- 

 midway between the two extreme types, more and more glass 

 developing as the traverse goes down hill from the top of an outcrop. 



In conclusion 1 would gratefully thank Professor E. W. Skeats 

 for his help and valuable suggestions throughout the prosecution of 

 the work ; also for his very kind criticism of this paper. I would 

 also like to thank Dr. 11. S. Summers for many discussions on 

 debatable points thai frequently arose. 



