2<S4 W. BOWIE THEORY OF ISOSTASY 



speaker believes that the geoisotherms are considerably depressed as sedi- 

 mentation progresses, although he has no experimental evidence that 

 would lead him to this conclusion. 



The change of density could not possibly be due only to the usual ther- 

 mal expansion of the material. If the material of the column had the 

 cubical coefficient of expansion of 0.000038 per degree centigrade and 

 was raised to a temperature 300 degrees centigrade higher than it had 

 before, we should expect an elevation of the surface of only about 3,500 

 feet if the column is 60 miles in length. 



If we assume that an area has been elevated to an average of 9,000 feet, 

 an increase of about 3 per cent in the length of the column of 60 miles 

 must have occurred. It is seen that there must have been some other 

 process taking place in the column under a sedimentary area than merely 

 thermal expansion, and this process possibly is a physical or chemical 

 rearrangement of the elements of the material resulting in a decrease in 

 density. 



As erosion continues in an uplifted area the isostatic balance of the 

 block below the area is maintained by a horizontal flow of material into 

 the block at its lower end. This inflowing material is undoubtedly denser 

 than the material eroded from the surface. Let it be assumed that the 

 density is exactly 10 per cent greater. Then as 1,000 feet of material is 

 eroded from the surface a disk only about 900 feet thick is brought into 

 the column, and in consequence the surface has been lowered 100 feet. 

 It is seen from this that, in order to baselevel ajuountain system, far 

 more material must be eroded than that represented by material which 

 is above sealevel when erosion began. In order to lower an area 5,000 

 feet, approximately 50,000 feet of material has to be eroded from the 

 area, on this assumption. It has puzzled geologists in some of their in- 

 vestigations to account for the vast amounts of deposits over a very large 

 area during a single geological formation ; but when we realize that per- 

 haps ten times as much material is eroded from a mountain area as is 

 represented by a change in the average elevation of the area, we see that 

 it is not so difficult to understand the source of the sedimentary material. 



As the isostatic adjustment continues with erosion, the block under the 

 area of erosion is raised a number of miles above its former position — 

 that is, each particle of the block will have been elevated an amount ap- 

 proximately equal to the total thickness of the material which has been 

 eroded. Let us assume that there have been six miles of material eroded 

 from a mountain area, while the elevation of the surface has been de- 

 pressed about 3,000 feet. Then the particles of the block, on an average, 

 have been raised above their former position to the extent of about six 



