204 SCIENCE PROGRESS. 



at which it is erupted. Experiment shows that viscosity of 

 a molten lava increases with its silica-percentage, especially 

 when the latter exceeds fifty-eight or sixty, and is also 

 increased by an unusual amount of alumina and by even a 

 moderate proportion of potash. Accordingly we find that 

 acid lavas are very decidedly less mobile than basic ones, 

 and so Mow less readily and to smaller distances ; and 

 further, that certain intermediate lavas, rich in alumina and 

 potash, are remarkably viscous, as is illustrated by the 

 peculiar dome-like forms assumed by some trachytic and 

 phonolitic eruptions. Again, the water which all rock- 

 magmas contain must contribute to reduce their viscosity, 

 though it does not perhaps materially affect the relative 

 viscosity of different types of lavas. In subaerial eruptions 

 much of it doubtless escapes as steam before the lava 

 spreads out as a coutte, but in submarine outbursts the 

 necessary retention of the contained water may consider- 

 ably modify the conditions of flow. It seems probable that, 

 beneath a rapidly consolidated crust, a lava may travel 

 over the sea-floor more readily and to greater distances 

 than it could accomplish on land. In all such questions 

 the extremely low thermal conductivity of rocks, both solid 

 and molten, must be borne in mind. 



A temperature considerably above the melting-point is 

 another condition which must promote the fluidity of a lava. 

 In a rock-magma occupying a subterranean reservoir any 

 noteworthy superheating can, as Becker remarks, scarcely 

 be postulated. Unless the magma differs in composition to 

 a remarkable degree from the encasing solid rock, an acces- 

 sion of heat will melt a portion of the latter rather than raise 

 the temperature of the former. This is confirmed by the 

 fact that in almost all lavas crystallisation has begun prior 

 to extrusion. The relief of pressure incident to the ex- 

 travasation of the magma must theoretically, however, 

 bring about a superheated condition, and direct evidence 

 of this is furnished by the corrosion and resorption of the 

 contained intratelluric crystals. Other things being equal, 

 the degree of superheating will be proportional to the 

 depth through which the molten magma has risen ; and 



