Art. XVIII. — Obsidianites — Their Origin from a 

 Cheinical Standpo int. 



By H. 8. 8UMMEUS, M.Sc. 



(Governiuont Science Research Scholar, Geolugiciil Laboratory, 

 University of Melbourne). 



[Read 8th October, 1908]. 



This paper is written in order to bring together the various 

 analyses which have been made of obsidianites and allied sub- 

 stances, and an attempt is made to show what bearing the 

 chemical composition has on the various hypotheses that have 

 been advanced to account for the origin of these interesting 

 objects. There is no necessity for any description of form, or 

 mode of occurrence, of obsidianites, as this part of the subject 

 has been exhaustively treated by Mr. Walcott,! who also gives 

 numerous illustrations of the different types found. 



Chemical Composition. 



In searching through the literature on obsidianites, I have 

 only succeeded in finding the records of seven complete analyses, 

 and one of these, that of a Victorian specimen, published in the 

 Melbourne Exhibition Catalogue, 1866, and quoted by Mr. 

 Walcott, is much too inaccurate for purposes of classification, 

 and is omitted from the following table of analyses. Mr. 

 Walcott kindly supplied me with a broken obsidianite from near 

 Mt. Elephant, Victoria, and this has been carefully analysed by 

 Mr. G. Ampt, B.Sc.^ in the Chemical Laboratory of the Uni- 

 versity. Mr. Ampt has also analysed the material of three small 

 buttons from near Hamilton, Victoria. These specimens were 

 obtained for me by Professor Spencer, through Mr. C. French. 

 A third analysis has been made by Mr. Ampt of a button from 

 Lake Eyre District, South Australia. This button was one of 



1 Proc. llo.v. Soc. Victoria, vol. xi., n.s., pt. I., 1898. 



