334 Ernest W. Skeats 



The nature of obsidian. 



During the last century the significance of many of the terms 

 used by the older mineralogists and geologists has by a process of 

 evolution undergone change and revision. The change has usually 

 been from a vague and general definition to one of a more precise 

 and limited character. The progressive changes in the meaning 

 attached to the word obsidian illustrate this process. 



A century ago, and even down to 60 or 70 years ago, the black 

 compact glass called obsidian was believed to be definite in com- 

 position and to constitute a true mineral species. 



Later it was shown to be a volcanic glass of variable composition, 

 and then any volcanic glass was spoken of as obsidian. At a later 

 date the glassy form of basalt was distinguished by the name of 

 tachylyte, while the name of obsidian was reserved for volcanic 

 glasses of acid to intermediate composition. 



In recent years intermediate volcanic glasses have been distin- 

 guished either as trachyte-glass and andesite-glass, or as trachytic- 

 obsidian and andesitic-obsidian. Avhile the term obsidian, without 

 qualification, has been by petrologists restricted to the acid volcanic 

 glasses corresponding in chemical composition to the rhyolites and 

 acid granites. 



It is this modern definition as an acid volcanic glass which alone 

 should be applied to obsidian, and it is in this sense that I, in 

 common with other petrologists, understand the term. 



The present-day definition implies an acid volcanic glass of about 

 the following composition : — 



Most obsidians have a silica percentage of over 70, and it should 

 be noted not only that the iron oxides seldom exceed 3 per cent., 

 but that the alkalies are commonly in notable excess over the alka- 

 line earths. 



