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in great quantities, and, by its violent escape, blows the lava into 

 the condition of spray. The crystals are thus ejected in a whole or 

 fragmentary state while the molten mass takes the form of glass. 

 Owing to the fact that bubbles are developed in immense quantities 

 at the time of eruption, the glassy matter frequently takes the 

 form of thin and more or less curved plates separating the 

 different bubbles. The external surfaces of the fine, glassy dust are 

 concave, with cusp-like projections between contiguous concavities. 

 When the glassy mass merely froths up in consequence of the 

 development of bubbles, but is not blown into the condition of spray, 

 pumice is formed. The glassy dust above referred to precisely resembles 

 disintegrated pumice. 



