42 Physiology of the Kidney 



and the silt is deposited in the sea along their edges, the added 

 weight of this deposit causes the plastic basalt to flow be- 

 neath the land masses and to float them higher in the air. It 

 is these slow adjustments to maintain isostatic equilibrium be- 

 tween the continents and the oceanic floor that sometimes 

 cause abrupt movements of the land/^^ But all the earth- 

 quakes of historic time are trivial when compared with the 

 disturbances of the past, which have extended not over days 

 or weeks, but millions of years. 



As measured, quite accurately it is now believed, by the 

 radio-active clock within its rocks, the earth has had its pres- 

 ent cold and semi-solid form for about 1800 miUion years. 

 During this period it has been cooling and shrinking as a 

 whole, having decreased in diameter something between 200 

 and 400 miles. Under the stresses resulting from this cooling 

 process, and more particularly in consequence of the alter- 

 nate fusion and solidification of the basaltic crust, this shrink- 

 ing has been intermittent rather than uniform, so that at 

 recurrent intervals of roughly 30 million years the continen- 

 tal masses have been wrinkled and folded into great mountain 

 chains. During the intervening periods of geologic quies- 

 cence, the mountains raised by the preceding diastrophic 

 movement have been largely if not entirely worn away to 

 sea level by the slow erosion of wind and rain. Schuchert^^ 

 estimates that the total continental depth eroded in this man- 

 ner since the opening of the Paleozoic exceeds 75 vertical 

 miles, or more than twenty ranges of mountains like the pres- 

 ent European Alps or the American Rockies. 



These periodic revolutions, as the geologist calls them, have 

 made us what we are. Because they have changed the form 

 and size of the continents and seas and at times submerged 

 great areas of land beneath the water, because they have di- 



