THE EXTRUSIVE PROCESSES. ' 635 



Thermal considerations. — Probably the most important consideration 

 relates to the heat effects. If underground- waters enter the lava- column 

 and come forth as steam, great quantities of heat are consumed in the 

 process. Has the lava a sufficient excess of heat to stand this? Can 

 ebullition be maintained for the observed periods if the steam comes 

 from ground-waters? 



Many lavas probably do not carry a very large excess of heat above 

 that necessary for liquefaction, for not a few of them contain crystals 

 already forming, which shows that they are within the range of the 

 temperatures of solidification of their constituents. The same con- 

 clusion is indicated by the limited fusing effects shown by the walls of 

 dikes and sills. On the other hand, as already remarked, dikes and 

 sills often show the effects of a rather rapid cooling from the walls. 

 The method of flow often imphes the same condition for the acidic 

 lavas, since they usually behave as stiff, pasty masses of limited hquidity. 

 On the other hand, the basic lavas, whose fusion-point is much lower, 

 often flow freely and reach great distances before solidifying. The 

 facts taken altogether imply that the average temperature of the lavas 

 is not much above the fusing-point of the acidic lavas, while it ma}^ 

 probably be very considerabl}^ above the fusing-point of basalt. For 

 a rough estimate, with no pretensions to accuracy, it may be assumed 

 that in an average case there are 500° Fahr. excess, but probably not 

 1000° Fahr. A computation based on even so rough an estimate as 

 this may, by showing the order of magnitude of the thermal consider- 

 ations, indicate their radical bearing. The average temperature of 

 the ground-water of the upper two or three miles of the crust — the 

 only portion through which water probably penetrates with sufficient 

 freedom to be effective in this case — is probably less than 200° Fahr. 

 The specific heat of rock appears to average somewhat less than 0.2. 

 The temperature of the lava may be taken at 3000° Fahr. as a suffi- 

 ciently high average. From these data it follows that if an amount 

 of ground- water equal to five percent, of the volume of the lava 

 entered the lava and was brought up to its temperature and then 

 discharged, the temperature of the whole mass would be lowered 550° 

 Fahr. It is therefore evident that only a small percentage of surface- 

 water can pass through the lava consistently with its continued 

 fluidity. 



M. Fouque estimated that the discharge of steam from a merely 



