216 J. P. Iddings — Origin of Quartz in Basalt, , 



ties into crystallized minerals, the former preventing such an 

 arrangement. 



Moreover if. as Professor A. Lagorio has done in a recent 

 paper <T. M. P. M.. vol. viii, 1887, 421) we consider rock mag- 

 mas as saturated solutions of silicate salts, and apply to them 

 the law which Sorby deduced for aqueous solutions of salts, 

 namely, that the solubility of those salts, which like the sili- 

 cates expand upon solution and condense upon crystallization, 

 is decreased by increasing pressure ; or, in other words, that 

 increasing pressure tends to crystallize such salts from solu- 

 tion ; then an increase of pressure alone would induce the 

 crystallization of certain silicate minerals from a molten magma, 

 or might lead to the crystallization of the whole magma. 



Unstable consolidation. — According to these views of the 

 effect of pressure on the viscosity of magmas and on their 

 crystallization, we should expect that an increase of pressure 

 would lead to the consolidation of magmas at temperatures 

 above their melting point for lower pressures. And we might 

 therefore have a highly heated magma within the earth under 

 such pressure that it exists as a solid mass, which may be 

 either crystallized or amorphous, or partly crystallized and 

 partly amorphous. Such a condition would be one of unstable 

 consolidation. 



Influence of water vapor — Another agent or force undoubt- 

 edly plays an important part in the liquefaction as well as in 

 the crystallization of heated magmas. The influence of water 

 vapor on the viscosity of lavas has been suggested long ago by 

 Scrope and others, and of its part as the explosive agent in 

 volcanic eruptions there can be t little doubt. Its presence in 

 larger or smaller amounts in almost every volcanic rock has 

 often been demonstrated. 



Eutectic substances. — The bearing of Dr. Guthrie's experi- 

 ments with cryohydrates or eutectic substances upon this 

 problem has been brought out by Professor J. W. .Judd in a 

 recent paper. (Geo! Mag., Jan., 1888.) The characteristic 

 feature of these mixed compounds is that their melting point 

 is considerably below that of the component substances ; they, 

 therefore, behave like alloys. Most of the substances experi- 

 mented with were hydrates, of which niter is chosen as an 

 example. Under ordinary conditions this substance melts at 

 320° C. upon the addition of 29 -07 per cent of water it melts 

 7 '6 C 0. Hence a mass of niter within the earth would be 

 solid at 300 a C. but at the same temperatures upon the acces- 

 sion of 15 or 20 per cent of water it would be molten, or in a 

 condition to become molten if the pressure did not prevent it. 

 Professor Judd calls attention to the power of the water of 

 hydration to lower the melting point of zeolites and siliceous 



