SHAPING THE EARTH BOWIE 329 



has a smaller density than that of the basalts which underlie the 

 oceans. Originally the earth must have had the granite or light 

 material lying over its surface like a huge blanket of fairly uniform 

 thickness. Why is it that now the granite is absent from such large 

 portions of the earth's surface'^ There are certainly no known 

 forces that could push the granite up into isolated masses. Gravity 

 would have resisted such piling up, and if forces had been sufficiently 

 great to force the granite into separate masses, these masses of 

 crushed rock would have slumped down soon after the forces had 

 ceased to operate. 



ISOSTASY 



It was a geologist, the famous C. E. Button, of the United States 

 Geological Survej', who coined the word " isostasy " in an address, 

 entitled " On Some of the Major Problems of Physical Geology," at 

 a meeting of the Philosophical Society of Washington, in 1889. 

 Dutton discussed some of the major problems of geology, including, 

 of course, the formation of mountains and the effect of the tre- 

 mendous amount of erosion and sedimentation. He came to the 

 conclusion that the shifting of material caused stresses which could 

 not be withstood by the strength of the earth's materials. He felt 

 that there must be a sagging down of the earth's surface under the 

 weight of the sediments and a rising up of the surface where erosion 

 had carried material away. He stated that in his opinion moun- 

 tains are not extra loads added to the earth's crust but that they are 

 due to lighter than normal material in the crust below them. In 

 effect he outlined what might be called a flotation hypothesis, that is, 

 that the continents were floating in heavier material just as ice floats 

 in water, A corollary of this hypothesis of Button's is that the 

 irregularities of the earth's surface are due to deviations from normal 

 densities in the outer portion of the earth. Under the oceans the 

 density is greater and under the continents less than normal. 



At the beginning of the present century geodesists realized that 

 isostasy w^as a subject of vital interest to them. Previously, for 

 decades, they had been attempting to explain the abnormal behavior 

 of the plumb line to which astronomical observations are referred 

 and of the pendulum by which values of gravity are determined. 



THE FIGURE OF THE EAUTH 



If the earth's surface had no irregularities but conformed to a 

 mathematical surface (a spheroid of revolution), then at any place 

 on it the direction of gravity would be at right angles to a plane 

 tangent to this spheroid at the point of observation. But the earth 

 has an irregular surface and due to this irregularity the figure 



