70 EVOLUTION OF THE EARTH 



Periodic reclevation of the lands. We have thus far pre- 

 sented the sedimentary evidence in the main as if it had con- 

 tinuously and regularly formed, without stoppages or "breaks." 

 The geologic record, however, was not laid down in this way, 

 because the surface of the earth does not remain stationary. 

 Even though to humanity the "everlasting hills" appear to 

 be permanent features of the earth's surface, they are all 

 doomed to be carried away by the rain and transported by the 

 rivers into the seas and oceans. Erosion of the land goes on 

 until all is worn to near sea-level, and then there is but a very 

 sluggish run-off of the rain that falls upon the planed con- 

 tinents. The earth has repeatedly had such a low relief; in 

 fact, this has been the condition during the greater part of 

 its history. 



The continents are repeatedly reelevated in relation to the 

 strand, and this goes on many times in a small way and less 

 often on a large scale (see Fig. 3). We have said that in the 

 course of the geologic ages the earth has shrunk at least 200 

 and possibly 400 miles in diameter. The earth is still shrink- 

 ing all the time, and this gives rise on the surface to small 

 warpings whose difference of elevation is usually not more than 

 a few hundred feet. These are but surficial symptoms, due to 

 internal readjustment, remarkable as it may seem, in an earth 

 ate rigid as steel local accommodations of the earth's mass 

 to loss of heat and molecular rearrangements. These altera- 

 tions finally set up strains in the lithosphere which are too 

 great for its strength to bear, and then there is a time of 

 breaking and greater readjustment between the relatively 

 settling and rising masses. At these times ranges of moun- 

 tains are slowly raised up near the margins of the continents, 

 due to the shortening, folding, and breaking of the earth's 

 crust, and the ranges have lengths of between 1,000 and 1,500 

 miles. These are the minor shrinkage movements, the "dis- 

 turbances," which are coming more and more to be regarded 



