Ohsidianites. 435 



All the analyses of acid and intermediate rocks from South 

 Victoria Land recorded by Dr. Prior are found to be closely 

 related to one another, and the analysis quoted may be taken 

 as fairly representative. 



Very few analyses of recent volcanic rocks from the Malay 

 Archipelago are recorded, the one given above being the only 

 superior analysis of material from this area, quoted by 

 Washington. 



An examination of the foregoing results should prove con- 

 clusively that there is no chemical relationship between the 

 ohsidianites and the rocks quoted, and therefore the chemical 

 evidence at present available is entirely opposed to the possi- 

 bility of New Zealand, South Victoria Land or Malay Archi- 

 pelago being the sources of the ohsidianites. 



Although no reliable analyses of Victoria basalts have been 

 recorded, numerous micro-sections have been examined, and 

 there is no evidence of any departure from a normal composi- 

 tion, and as they are all basic in character, the ohsidianites 

 cannot possibly be glassy representatives of these rocks. Mr. 

 Duim^ suggests that the ohsidianites may have preceded the 

 basaltic flows, but this is impossible in some cases, as the but- 

 tons are often found resting on the surface of the lava flows. 

 In some areas it has been shown^ that a gradual change has 

 taken place in the composition of the lava poured out by the 

 volcanoes of that area, the result being that whereas the earliest 

 flows were basaltic, the final products were more closely allied 

 to obsidian. 



Even if we disregard the almost unique composition of the 

 ohsidianites, we are not justified in assuming that they repre- 

 s; nt any such acid residuum froiu a basic magma, for if acid 

 differentiation products wei'e formed we should certainly find 

 some traces of them in or around some of the extinct cones. 



It would seem, therefore, that the advocates of a volcanic 

 origin for the ohsidianites receive no support from a considera- 

 tion of their chemical composition. 



1 Records of the Geo). Survey of Victoria, vol. ii., yt. IV., 1908, p. 204. 

 1 Geikie's Text Book of Geology, pp. 137, 34!), 70s. 



