286 ANNUAL REPORT SMITHSONIAN INSTITUTION, 192 8 



density differences in the lithosphere and the maintenance of the 

 heated'^ and weak condition in the asthenosphere. 



Uranium, in degenerating through radium to the stable element 

 lead, develops an enormous amount of heat, but at so slow a rate that 

 the whole duration of geologic time has not sufficed to eliminate it 

 from the crust of tiie earth. Uranium and thorium, the parents of 

 the radioactive series, are widely though sparsely diffused through 

 the lithosphere. If they extend with their amount at the surface 

 down to a depth of 40 miles," they must supply heat to the surface 

 as fast as it is lost by radiation into space. 



The small content of radioactive elements in the basaltic shell or 

 asthenosphere below the granitic crust of the continents would then 

 supply that slow increment of heat w^hich is necessary to generate 

 new molten rocks. The granitic shell loses its excess heat by con- 

 duction to the atmosphere, but the asthenosphere is so deeply buried 

 that its heat can not escape, but must slowly transform some of the 

 solid rock into liquid form. Keservoirs of molten rock gather until 

 their mass, combined with their decreased density in the fluid form, 

 enables them to work their way into and through the lithosphere 

 and demonstrate their existence in igneous activity at the surface 

 of the earth. The magma which thus comes from the greatest depth 

 and in greatest volume would, because of the initial density stratifica- 

 tion, produce a notable increase in the density of the outer crust. In 

 order to reestablish isostatic equilibrium, such a region must subside. 



Most of the igneous rock of later geologic ages which has been 

 intruded into the outer crust beneath the present continents clearly 

 has not increased the density sufficiently to produce a foundering and 

 would appear therefore either to have come from somewhat higher 

 levels or to have risen in smaller quantity. In some regions, how- 

 ever, as in that of the Lake Superior Basin, large masses of basic 

 magma do seem to have overweighted the crust in an early geologic 

 period and produced a tendency to settle as a basin. The same effect 

 may have taken place to even a larger degree in some regions of 

 notable recent subsidence, as in the Mediterranean Basins. In the 

 earliest times, following the solidification of the earth, the forms and 

 relations of the ocean basins suggest that dense molten matter from 

 the depths of the earth broke into or through the outer crust, on a 

 gigantic scale, eruption following eruption until the widespread 

 floods had weighted down broad areas and caused their subsidence 

 into ocean basins. 



As seen in the lava plains of the moon, such an action, once started 

 at a certain point, is conceived to have gone forward with widening 

 radius, leading to the origin of the many rudely circular outlines 



"This fisiirf is greatly reduced by recent estimates. See Editorial Preface, with foot- 

 notes. 



