May, 1922 dynamics of the lithosphere 205 



Vol can ism P — -The evidence of geographic distribution, extent 

 of extrusion, and differentiation of magmas indicates that volcan- 

 ism is a local phenomenon. Seismic evidence clinches the 

 argument.™ Distortional waves will not pass through a liquid, 

 yet they pass with perfect freedom through any part of the 

 crust at which observations have been made. There cannot, 

 therefore, be any considerable lava lakes beneath the earth's 

 surface. Isostatic investigations throw an illuminating line of 

 evidence on the problem. Igneous intrusions and extrusions 

 of all rocks are almost perfectly compensated.'*^ It would seem 

 then that volcanism is simply one method of quick adjustment 

 to the equilibrium figure. It would be found in places of defective 

 density w^here decreased pressure produces liquefaction in the 

 form of a vesicle which rises to the surface, and in one way or 

 another is ejected by force. Most of the vesicles stop before 

 they reach the surface. Denser material is thereby added to, 

 or replaces, lighter material bringing about isostatic compensa- 

 tion. The method would appear to be thoroughly delicate to 

 reach isostatic adjustment so perfectly. One would expect the 

 process once set off to continue this inertia beyond the point of 

 perfect compensation. This is known to occur frequently in the 

 case of volcanic islands that are thrown up with a positive 

 anomal}^ but which soon sink into a state of equilibrium. 



Moimtain Making. — This is another process resulting from 

 the tendency of the crustal substance to maintain a state of 

 equilibrium in reference to the figure of the earth through the 

 force of gravity. The process is this: Marine sediments are laid 

 down in a geosyncline, or along the edge of a continent. To 

 maintain adjustment subsidence takes place until great depths 

 of sediments are found. Isostatic adjustment is practically per- 

 fect throughout the process. The sediments thus laid down 

 have, of course, a lesser density than the sediments or rocks on 

 either side. There is, therefore, an imperfect horizontal adjust- 

 ment to the bottom of the geosyncline. Now% as the sediments 

 are forced downward temperature rises. Probably 800 degrees 

 is attained at the bottom of the geosyncline, and since it con- 

 tains rocks w4th more or less water inclusion, the steam devel- 

 oped, or at least the heated water when pressure is too great 



^^Iddings: Problem of Volcanism. 



'"Smithsonian Rpt. (1916), The Earth's Figure, etc. 



"Special Rpt. No. 10, U. S. C. & G. Survey. 



