PLUTONIC ROCKS . 



Granites (23) 

 Syenites (10) 

 Gabbros (13) 

 Peridotites (5) 



VOLCANIC ROCKS. 



CRYSTALLINE OR SEMI-CRYSTALLINE. 



Felsites and Liparites (38) ... 2'61 

 Porphyrites and Andesites (23) ... 2-66 

 Basalts (23) 2'86 



2-532-70 72-25 

 2-542-79 59-20 

 2-753-10 48-36 



76-0667.61 

 66-7554-73 

 53-7342-65 



Liparite-glass (11) 

 Andesite-glass 

 Basalt-glass (9) ... 



VITREOUS. 



2-36 



2-262-41 73-80 82-8071-19 



2-71 2-692-76 52-32 54-9350-82 



An examination of this table suggests the following conclusions : 



The specific gravity decreases as the silica-percentage increases. 



The specific gravity decreases as the total amount of glass entering 

 into the composition of the rock increases. 



It must be remembered, however, that the law connecting specific 

 gravity and silica-percentage is only true in a broad and general sense. 

 It fails when applied to minute differences and to individual cases, although 

 it stands out strongly when averages are taken. Deviations from the law 

 are due partly to the fact that the silica-percentage does not govern 

 absolutely the relative proportions of the other constituents, and partly to the 

 fact that rocks having the same chemical composition frequently differ in 

 mineralogical composition. 



It is interesting to compare terrestrial with extra-terrestrial rocks 

 (meteorites) so far as specific gravity is concerned. For this purpose 

 meteorites may be conveniently divided into four groups. 



(1) This table has been compiled by taking a number of cases at random from " ROTH'S 

 Beitrage sur Petrographie der plutonischen Gesteine." The number taken in each case is 

 indicated by the figures in brackets. 



