BY W. H. TWELVETREES AND W. F. PETTERD. 25 



we know, to this part of Tasmania. Its unexpected dis- 

 covery may be placed to the credit of the young and 

 expanding science of microscopical petrology.* 



* Since writing the above we have seen Prof. Rosenbusch"s new work 

 on the Elements of Petrology (Elemente der Gesteinslehre. 1898), in 

 which he groups the trachytes and quartzless jDorphyries (orthoj^hyres) 

 in one family. He says, (pp. 265-6), " Orthophyres differ from trachytes 

 only in their greater age and consequent inferior preservation, viz.. in 

 the more frequent red and brown color, diminished porosity of the 

 ground-mass, dull aspect of the felspars and extreme decomposition of 

 the colored constituents. Fresh orthophyres cannot be distinguished 

 from trachytes." Referring to the sanidine in orthophyres, he adds. 

 (p. 266) : — '• In the quartzless porphyries sanidine has mostly, though 

 by no means always, surrendered its glassy habit, and possesses the 

 habit and often the i;ed color of orthoclase." 



