89 



MIOEOSCOPICAL STUDIES OF TASMANIA^ 



EOCKS. 



By W. H. Twelveteees, F.G.S. 



Bead May 28th, 1896. 



The delicate methods of modern microscopical petrology- 

 have added greatly to our knowledge of igneous rocks. Very 

 little work of this description has hitherto been attempted in 

 Tasmania, but the present studies, undertaken in connection 

 with the Launceston Microscopical Club, have been entered 

 upon in the hope of throwing additional light upon the 

 genesis and intimate structure of our eruptive rocks. It is 

 proposed to continue the descriptions from time to time as 

 materials and opportunity permit, and as they must at first 

 necessarily partake of the nature of preliminary work, re- 

 marks upon their classification will be withheld till sufficient 

 progress has been made to justify generalisations. A glance 

 at the current literature of our science suffices to show that 

 the microscope in petrology is each year enlarging the 

 boundaries of its conquests, and it is hoped that the present 

 contributions will prove helpful to the stratigraphical 

 geologists of this island. 



I. The Dolerite [= Diabase of Authors] of the 

 Cataract Gorge, Launceston. 



The samples of rock selected for slicing were taken from 

 the quarry near the Cataract Bridge. Microscopically, the 

 stone is dark, granular, doleritic-looking, studded with 

 glistening needles of felspar, and black or dark-green spots 

 representing a ferro-magnesian mineral. Its specific gravity, 

 as determined by the use of a Walker's balance, ranges from 

 2*9 to 3, showing that we are dealing with a basic rock. 



From a microscopical examination of their sections, we 

 learn that the essential mineral components are a plagioclase 

 felspar, augite, and a little iron oxide. I have not found any 

 olivine in my sections, but further examples will have to be 

 examined with respect to this point. 



FELSPAR. 



The sections presented are lath-shaped, columnar, with a 

 few tabular forms. They are tolerably even in size ; at all 

 events, there is not that disproportion of size which would 

 suggest two generations. Though there are a few Carlsbad 

 twins, the felspars are mostly twinned on the albite plan, and 

 by choosing sections out of the zone perpendicular to the 

 brachypinacoid (010), we can obtain the extinction angle 

 proper to the species represented. With this method of 

 determination not many visible sections are useful, as it is 



