36 HAYFORD 



less than three miles — the average elevation, not the elevation 

 of the summits. Yet the present isostatic compensation, as 

 already stated, departs not more than one-third from present 

 perfection. The only reasonable explanation is that the iso- 

 static readjustment keeps pace approximately with erosion and 

 deposition. 



Upon the basis that the isostatic compensation is complete and 

 uniformly distributed throughout the first 71 miles of depth, 

 will the computed variations of density be so great as to raise a 

 doubt of the validity of the conclusions which have been drawn? 



The highest large area within the region covered by this in- 

 vestigation is the region southwest of Denver, Colorado, with 

 an elevation of about 11,000 feet or 2.1 miles. This is 3 per 

 cent, of 71 miles. Hence, on the basis stated, the average 

 density of the material beneath this region is 3 per cent, less 

 than that beneath the areas along the coast which lie practically 

 at sea level. The deepest ocean area of considerable size 

 within the region of the investigation is in the Atlantic, north- 

 east of the Caribbean Islands, with a depth of 3,000 fathoms 

 or 3.4 miles. On the basis stated the average density of the 

 material underlying this deep spot is only 3 per cent, greater 

 than that of the material under areas which lie at sea level. 



This computed variation in density is small, much smaller 

 than the variations in density between different rock samples 

 from different regions. Hence it presents no contradiction to 

 the supposition that the location of continents and oceans may 

 be due to initial differences of density in the materials. 



But if there is a continuous isostatic readjustment in progress 

 it is apparently necessary to believe that a given material may 

 change in density as much as 3 per cent., under the varying 

 conditions as to pressure (and possibly temperature) to which it 

 is subjected within the first 71 miles of depth in the earth. 



Both laboratory obvervations and geologic observations indi- 

 cate that this is not only possible but probable. 



The elastic effects probably cooperate in producing such 

 changes of density, but probably play a minor part only. 



Laboratory experiments have established as a general law of 

 chemistry that increase of pressure favors such chemical 



