342 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 31 



come up, while others have gone down, but I am of the opinion that 

 the average difference in elevation of the ocean beds and of the 

 continents, now about 2i/2 miles, has not been much less than it is now 

 at any time during the sedimentary period. 



We can now, I believe, get an idea as to where some of the force 

 originates which changes the configuration of the earth's surface. 

 The water falling as rain carries off vast quantities of material in 

 suspension and solution. It unloads certain portions of the earth's 

 crust and overloads others. Some geologists have told us that as 

 much as 30,000 feet, about 6 miles, of material have been eroded 

 from some mountain areas. Then there are other areas on which 

 as much as 40,000 feet, or nearly 8 miles, of sediments have been 

 placed. The earth's materials are not strong enough to resist yield- 

 ing under these great negative and positive loads. There is a bend- 

 ing down of the crust under the sediments and a deflection upward 

 of the crust under the areas which have undergone great erosion. 



The movement of material within the first 5 or 10 miles resulting 

 from the loading and unloading by erosion and sedimentation is 

 not a simple one. We do not have merely a slab of material which 

 can break or bend, but a shell approximately 60 miles in thickness 

 completely encircling the earth. Any distortion or change in one 

 part of this shell would have an effect on all other parts of it if the 

 earth's crust were of tremendous strength, but such is not the case. 

 The crust must yield under comparatively small amounts of sedi- 

 mentation and erosion. If this were not true, the geodetic data 

 Avould certainly enable us to detect without difficulty the extent 

 of the masses involved. An extra load of 1,000 feet of material over 

 the Rocky Mountain area would show up at once in the gravity 

 data. The absence of any large differences from normal conditions 

 leads us to believe that there is surely no excess or deficiency of ma- 

 terial for the whole Rocky Mountain region equivalent to a blanket 

 1,000 feet in thickness. A blanket of even 500 feet of material is 

 greater than can be present as an undetected excess or deficient load 

 for an extensive area. We therefore may conclude, I believe, that 

 a blanket of surface rock 500 feet in thickness over a large surface 

 area exerts a force that is great enough to make the crust beneath 

 yield. This yielding at times is so slow that the rocks will merely 

 be bent and deformed, and at other times it is so rapid as to cause 

 rocks to break. 



THERMAL CHANGES IN CRUST 



Isostasy is a condition of rest. Wlien the materials of the earth's 

 surface are carried in great amounts from one area to another during 



