JR. A. Daly — Mechanics of Igneous Intrusion. 27 



from 20° C. to molten condition at 1200° C. is possibly only 

 about 13 per cent, instead of about 16 per cent, as noted in 

 the first paper." 



Quite recently J. A. Douglas has made a number of very 

 careful measurements of the densities of typical igneous rocks 

 and of their respective glasses, all specific gravities being 

 taken at room temperatures.! . Douglas's method is reliable 

 and his results accordant. For gabbro he found the decrease 

 of specific gravity, in passing from rock to glass, to be 5*07 

 per cent. Delesse had found the decrease to be 11*46 per 

 cent, as the average of measurements of two specimens from 

 different localities Barus's determination, 10 per cent, is 

 intermediate between the two. 



It seems probable, therefore, that a decrease of 6 per cent in 

 specific gravity (rock to glass at 20° C.) is close to the minimum 

 for the average gabbroid rock, and it is possible that Barus's 10 

 per cent decrease is too high for average gabbro. For present 

 purposes it is safer to use the minimum value of 6 per cent. 

 Similar minima for diorite (6 per cent), quartz diorite and 

 tonalite (7 per cent), syenite (8 per cent) and granite (9 per 

 cent) have been estimated from the numerous measurements of 

 Delesse, Cossa and Douglas. Each of these rocks certainly 









Table 



II. 









- 



Specif] 



.c gravity of 



Specific gravity 



of same 



rock 





Crystalline rock at 





when molten at 



A 







20°C. 1000 C C. 



1300°C. 



1000°C. 



1100 o C. 



1200°C. 



1300°C. 





f 2-80 



2-73 



2-71 



2-57 



2-56 



2-54 



2-53 



Gabbro 



| 2-90 



2-83 



2-80 



266 



265 



2-64 



2'63 



and 



\ 3-00 



2-92 



2-90 



2-75 



2-74 



2-73 



2'72 



diorite 



| 3-10 



3-02 



3-00 



2-84 



2-83 



2-81 



2-80 





[3-20 



3-12 



3-10 



2 94 



2-92 



2-91 



2-91 



Quartz-dio 

















rite and 



j 2-70 



2-63 



2-61 



2-46 



2-45 



2-44 



2-43 



tonalite 



\ 2-S0 



2-73 



2-71 



2-54 



2-53 



2-51 



2-51 





( 2-60 

 \ 2-70 



2-54 



2-52 



2-33 



2-32 



2-31 



2-31 



Syenite 



2-63 



2-61 



2-42 



2-41 



2-40 



2-40 





[ 2-80 



2-73 



2-71 



2-52 



2*51 



2-50 



2-50 



Granite 



( 2-60 



2'54 



2-52 



2-31 



2-30 



2-29 



2-29 



and 



\ 2-70 



2-63 



2-61 



2-40 



2'39 



2-39 



2-38 



gneiss 



( 2-80 



2-73 



2-71 



2'49 



2'48 



2-47 



2-47 



^Bischof, in 1841, found that basalt expanded 7 per cent in passing to a 

 glass at room temperature, and 10 4 per cent in becoming molten. (Quoted 

 from Zirkel's Lehrbuch der Petrographie, 2d eel., 1893, vol. i, p. (583.) 



f Quart. Jour. Geol. Soc, xiii, p. 145, 1907. 



