274 M. A. Daly — Mechanics of Igneous Intrusion. 



the glass resulting from the rapid cooling of the fused product, 

 both glass and molten rock being again under one atmosphere 

 of pressure. His results are summarized in the following table 

 and list of conclusions from his own more detailed scheme of 

 observations and from his important fusion curve. As it is 

 expedient in the present connection to choose a reference tem- 

 perature that will be high enough to allow of a degree of 

 superheating of any plutonic rock-type sufficient to keep it 

 molten even under several thousand atmospheres of pressure, 

 the writer has chosen 1400° C. as the basis for comparison of 

 the densities of molten magmas. 



Table I. 

 Temperature. Volume ratios. Specific gravities. 



Rock 20°C. 100 3-0178 



Glass 20° 111*2 2-7L7 



Molten rock 1400° 119-6 2*523 



Increase in volume in passing from glass at 20° C. to 



molten condition at 1400° C . . 7*7 </o 



Increase in volume in passing from glass at 20° C. to 



melting point at about 1090° C. _.. 2*8$ 



Net decrease in specific gravity in passing from rock 



at 20° C. to glass at 20° C _ 10$ 



Decrease in specific gravity in passing from glass at 



20° C. to molten condition at 1400° C 7*1 $ 



Decrease in specific gravity in passing from rock at 20° 



C. to molten condition at 1400° C. _ 16*4$ 



Decrease in volume in the act of solidifying to a glass, 3*9 $ 



There was a notable and sudden increase in volume at melt- 

 ing. The expansion was steady up to the melting point and 

 again steady, but more rapid after melting than before, being 

 about 1*9 times faster. 



Barns also gives a timely explanation of the reason why a 

 fragment of solid diabase will not readily be made to sink in 

 the molten glass of its own substance. He shows that a " boat," 

 made of chilled glass, is instantly formed about the cold frag- 

 ment as it meets the surface of the fused mass. A full 

 corroboration of his conclusion that such a fragment, once 

 completely submerged in the non-chilled molten glass, must 

 forthwith sink, has been given by Lagorio's experiments.* 

 They also correspond with observations by Johnston-Lavis, who 

 has seen compact lava sink in a liquid flow from Yesuvius.f 



A critical comparison of other experimental studies with 

 those of Barus indicates that they fully bear out his conclu- 



*Tscher. Min. u. Petrog. Mitth., viii, 510 (1887). 



f Quart. Jour. Geol. Soc. , xxxviii, Proc. , 240 (1882). Cf . Dutton at Kilauea, 

 4th Ann. Rep. U. S. Geol. Surv., 106 (1884). 



