278 THE CANADIAN NATURALIST. [Vol. vi. 



It will be observed from this table that the specific gravity of 

 granular rocks is generally greater than that of the trachytio 

 rocks which correspond with them in degree of acidity ; granites 

 are heavier than rhyolites, and greenstones than dolerites. (The 

 rule does not hold good when applied to the basic rocks, but this 

 may be owing to the facility with which they become decomposed 

 and absorb water, which causes a material diminution of gravity.) 

 The porphyritic rocks seem to occupy a position between the 

 other two series, being neither so dense, relatively, as the granular 

 nor so light as the trachytic rocks. This would seem to indicate 

 that the coarsely granular rocks crystallised more slowly and 

 perfectly than the porphyries and the latter more than the tra- 

 chytes. This diflPerence in density between rocks having the 

 same percentage of silica is even more observable between 

 trachytic and vitreous rocks. Obsidian has invariably a much 

 less specific gravity than a quartzose trachyte which possesses the 

 same percentage of silica. Thus we have the specific gravity of 



Khyolite from Palmarola with 74.54 p. c. Si. Og = 2-529 

 Obsidian from Lipari with 74.05 " = 2.370 



Quartz trachyte from Besobdal, 



Asia Minor, with 76.56 " =2.656 



Obsidian from Little Ararat with 77.27 " =2.394 



The cause of the difi"^rence se^ms merelv to be that while the 

 rhyolites cooled slowly and shrank together to a denser mass, the 

 obsidians are quickly cooled unannealed natural glasses. It is 

 well known that garnet, vesuvianite, orthoclase, labradorite, 

 augite, and olivine have their densities much decreased by being 

 fused and quickly cooled, and the same thing has been remarked 

 with regard to rocks. St. Claire Deville, and Delesse experi- 

 mented on several rocks, and found that their specific gravities 

 were diminished after fusion. St. Claire Deville's results were 



as follows : 



Specific Specific 



Gravities Gravities 



before fusion, afterfusion. 



Vitreous lava from the Peak of Tenerifte 2.570 2.464 



Trachyte from Chahorra 2.727 2.61 7 



Basaltic lava from the Peak of los Majorquines 2.945 2.836 



Basalt from Pic de Foga, Cape of Good Hope.. 2.971 2.879 



Granite from Andoux 2.662 2.360 



Delesse found the loss to be less with fine-grained and semi- 

 vitreous rocks than with those of a distinctly crystalline character. 

 According to his results, if the rocks experimented on be arranged 

 according to the degree of diminution which their specific gravities 

 undergo in fusion, beginning with those which experience greatest 



