REPORT OF THE CHIEF ASTRONOMER 



743 



SESSIONAL PAPER No. 25a 



Summarizing all the results we have : — 



Table L. 

 Decrease in density (rock to glass at 20° C). 



Diabase of Barus 10-00% 



Gabbro of Douglas 5-07 



Average gabbro of Delesse 11-57 



Average diorite of Delesse 7-07 



Average diorite of Douglas 5-65 



Quartz diorite of Cossa 9-90 



Syenite of Cossa 10-33 



Syenite of Douglas 6-02 



Tonalite of Douglas 6-87 



Average granite of Douglas 8-78 



Average granite of Delesse 9-16 



Gneiss of Delesse 6-95 



Average of all above , 8-8-5 



Average of seven minerals (Carnegie Geophysical Laboratory) . . 10-6 



We may conclude that the acid rocks certainly expand more" than the basic 

 ones in passing to the glassy state at room temperature. It is probable, though 

 not certain, that the expansion of the more acid glasses with heating is not 

 much more rapid that that of the basic glasses. In any case, we shall, in the 

 following argument, make no mistake in principle if we assume that all the 

 leading types of crystalline rocks expand at least as much as gabbro (or diabase) 

 when molten at the high temperature of 1300° C. 



Reade in a large number of determinations found that rock expands, on the 

 average, at sensibly the same rate as that found by Barus for diabase, namely, 

 about 0-000025 volume per degree Centigrade.* Using this figure, allowing for 

 the various rates of decrease in density for the different rocks in passing into 

 the glassy condition, and assuming that each glass expands, with heating, at the 

 same rate as Barus's diabase, we have the data of Table LI.: — 



TABLE LI. 



Specific gravities of rocks and melts. 





Specific gravity of ci 



ystalline 



Specific gravity of same rock when 







rock at 







molten at 





20 C C. 



1000°C. 



1300°C. 



1000°C. 



1100°C. 



1200°C. 



1300°C. 





r 2 



80 



273 



2-71 



2-57 





56 



254 



253 



* 



2 



90 



2 83 



280 



2-66 



2 



65 



2-64 



263 



Gabbro and diorite . . 



\ 3 



00 



2 92 



290 



2-75 



2 



74 



273 



2'72 





3 



10 



302 



300 



2-84 



2 



83 



2-81 



2-80 





I 3 



20 



312 



310 



294 



2 



92 



291 



291 



Quartz-diorite and 

 tonalite 



f 2 



70 



263 



2-61 



2-46 



2 



45 



244 



243 



I 2 



30 



273 



271 



254 



2 



53 



2; 51 



251 





I 2 



60 



254 



2-52 



233 



2 



32 



231 



- 2 31 





I 1 



70 



2 ■ 63 



2 61 



2 42 



2 



41 



2-40 



2"40 





I 2 



80 



273 



271 



252 



2 



51 



2-50 



2-50. 





f 2 



I 2 



60 



2-54 



2-52 



231 



2 



30 



229 



229 



Granite and gneiss. . . 



70 



2 63 



261 



240 



2 



39 



239 



2 38 





80 



2 73 



2-71 



249 



2 



48 



2-47 



247 



* T. M. Eeade, Origin of Mountain Ranges, 1886, p. 110. 



