EARTH 7 



melting and expansion of the subcmst, perhaps in local- 

 ized areas of convection, with resulting elevation of the 

 overlying rocks, then the heat is lost from the interior 

 by volcanic action and other routes, this process giving 

 rise to periodic 'blistering' of the surface. Under the tidal 

 pull of the sun the continents have tended to drift west- 

 ward, and it still is beheved by some that at an early 

 time there was one great land mass that split into frag- 

 ments which, drifting in periods and areas of high mande 

 temperatures, produced the continents as we now see 

 them. These continents even now are under great strain 

 which is relieved by fracturing of the thinner parts of 

 the crust, especially under the oceans, permitting extru- 

 sion of material from the interior and movements of the 

 exposed land masses into new positions of equilibrium. 

 Possibly the earth has contracted during its planetary 

 history by as much as 200 to 400 miles, wrinkling the 

 crust like the skin of a drying apple. 



Whenever land masses were elevated, frost, ice, sun, 

 wind, and rain began their work of wearing them down, 

 weathering them first to boulders and pebbles, and fi- 

 nally to silt that the rivers carried into the sea and de- 

 posited on the ocean bottom along the continental ledges. 

 This silt, in places accumulating to a depth of many 

 miles, added its weight to that of the overlying water 

 and pressed down on the underlying basalt and forced 

 it under the lightened continental masses, floating them 

 higher above the sea. (A fair idea of the magnitude and 

 speed of erosion is afforded by the facts that the Colo- 

 rado River carries annually into the sea an average of 

 184 milhon tons of silt; the Mississippi, an average of 

 730 million tons.) 



In any case, the sequence of mountain building fol- 

 lowed by erosion has gone on throughout earth's history, 

 not in a uniform manner but with marked intermittency. 

 Erosion is rapid when mountains are highest, and slow 

 or negligible during periods of low continental relief, and 



