CONCLUSION 299 



the Coast and Geodetic Survey, who shows that only a small percentage 

 of the depression could be due to elastic deformation, leaving the greater 

 part to be accounted for in some other way. 



There are many other possible processes which might be discussed with 

 profit, such as the change in crystal form of rock-forming material under 

 varying conditions of heat and pressure and the expulsion under certain 

 conditions of such mobile material as water and gas ; but, as I have not 

 been able to find much quantitative data bearing on these problems, dis- 

 cussion of them is omitted. Other significant problems have to do with 

 possible fluctuations of sealevel. 



It is well known that sealevel may rise and fall as the capacity of the 

 ocean basins changes, and that altitude as reckoned from sealevel may 

 change without any movement of the land whose altitude is measured. 

 Doubtless some of the apparent subsidence of the Cretaceous Basin was 

 due to rise in sealevel. It has also been pointed out that material eroded 

 from the uplands and transported to distant regions may affect the posi- 

 tion of sealevel, under the well-known principle that a landmass tends to 

 draw the sea water toward it. If the rocks of the continents and basins 

 were of equal density and the landmasses above sealevel be reckoned as 

 excess material, the rise of sealevel on the flanks of highlands would be a 

 large factor in determining apparent altitude; but under the h3^pothesis 

 of isostatic adjustment, which calls for equal mass beneath highlands and 

 basins, the change in sealevel because of land attraction is found to be 

 only a few feet. According to computations kindly furnished by Mr. 

 Lambert, a continental mass in isostatic adjustment will affect sealevel 

 so little that, for purposes of this paper, the effect may be neglected. The 

 approximate upwarp of the water surface at the middle of a continental 

 mass of average density, 2.83, is as follows : 



Elevation of continent above 

 mean sealevel. 





Wai-ping of geoid under complete 



compensation. Depth, 100 



kilometers, ov about 



62 miles. 



Feet 





Feet 



0, 





33 



5,000 





63 



10,000 





93 



15,000 



Conclusion 



123 



In conclusion, two critical considerations may be emphasized: First, 

 changes in density due to flow of heat are slow and they usually oppose 

 crustal movement, halting and finally reversing it. Second, a column of 

 .rock made light by increased temperature and decreased density may rise 



