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ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 64 



One of the reasons for this is that the accessible material is far less 

 than one-thousandth part of the whole mass of the Earth. In this 

 minute proportion, the geologist nevertheless finds signs of almost 

 every kind of disturbance that could be conceived, with the result that 

 almost any conjecture about the Earth's history can find some apparent 

 evidence for its support. This renders the task of the theorist both 

 difficult and thankless. Ho^Yever, one general conclusion has emerged 

 from this work, and that is that the outer crust of the Earth has under- 

 gone considerable horizontal shortening, as if to fit down on to a de- 

 creasing and shrinking interior. The problem is to find the cause of 

 this contraction. 



CONVECTION CELLS 



One obvious suggestion is that at one time the Earth rotated much 

 faster than now. This would have caused it to have bulged out far 

 more at the equator than at present. As the shape became less sphe- 

 roidal, this would lead to crumpling at the surface, presumably mainly 

 along meridional lines. There is certainly some evidence of a greater 

 rotation rate in the past, but this theory would place the greatest 

 changes in surface area far back in the Earth's history, wliereas moun- 

 tain building is still going on now, even though changes of surface 

 area due to changes of shape are negligible. 



An entirely different theory maintains that the mountains are pro- 

 duced by the drag of circulating convection currents actually flowing 

 in the Earth's solid mantle {see fig. 1) . It is considered that such cur- 

 rents would have a pattern, dividing off into a certain number of 

 convection cells just filling the volume of the mantle. From time to 

 time, as a result of the increase in size of the liquid core of the Earth, 

 the number of cells would have to increase by one. The drag on the 

 surface layers, if effective at all, would be producing mountains all 



Figure 1. — The diagram shows how circulating convection currents in the solid mantle 

 deep within the Earth are supposed to give rise to surface features. These convection- 

 cells are also supposed to account for Earth's drifting continents. 



