486 FRANK F. GROUT 
On the other hand, in as weak a solid as gelatine gas bubbles and 
small solids remain stationary indefinitely. Bowen’s work on the 
settling of crystalst may be taken as evidence that a magma during 
crystallization is truly viscous. In such a liquid, then, any appre- 
ciable force applied is entirely sufficient to start action. It simply 
remains to estimate possible counteracting forces and the rate of 
motion likely to result. 
It was estimated by Becker? that Hawaiian lavas were about 
fifty times as viscous as water (0.575 in C.G.S. units; water at 
15°,0.0115). Daly estimates that rhyolites may be erupted at a 
viscosity of 11.5. Ranging from these values for actively moving 
magmas, we may be sure that on cooling the viscosity increases until 
in glasses it is almost infinite. Data connecting the viscosity with 
temperature are not available. Fortunately Bowen has noted the 
rate of settling of certain crystals and thus obtained some very 
useful estimates for viscosity during crystallization,’ which is the 
condition of most importance rather than the actual temperature. 
His figures for probable maximum viscosities are 4 to 200 in C.G.S. 
units for melts ranging from basic to acid character, in which crys- 
tals are growing. Bowen calls these figures maxima because of 
the probable growth of the crystals during settling. Several factors 
tend to reduce the values in nature. The extreme fluidity actually 
shown' by intrusive magmas may be due to their retention of more 
water vapor than is the case in extrusive lavas. In agreement with 
this are the results of Morey® showing that fusions in the presence 
of steam show a remarkable decrease in viscosity. ‘The importance 
of water in magmas is attested by hydrous minerals and miarolitic 
cavities in the rocks. On the other hand, some conditions may 
increase viscosity. Doelter found that pressure increased it, but 
«N. L. Bowen, ‘‘ Crystallization Differentiation in Silicate Melts,” Am. Jour. Sci., 
XXXIX (1915), 186. 
2G. F. Becker, op. cit., p. 20. 
3R.A. Daly, “Mechanics of Igneous Intrusion,” A mer. Jour. Sci., X XVI (1908), 30. 
4N.L. Bowen, “Crystallization Differentiation in Silicate Melts,” Am. Jour. Sci., 
XX XTX (1915), 186. 
5A. Harker, The Natural History of Igneous Rocks, p. 223. 
6G. W. Morey, ‘‘New Crystalline Silicates of Sodium and Potassium,” Jour. 
Amer. Chem. Soc., XXXVI (1914), 226. 
