THE EARTH AS A WHOLE ii 



the pressures at the bases of the two columns are equal. But, since 

 the heights of the columns are unequal, it follows that their densi- 

 ties must also be unequal, the shorter column, !>, having the greater 

 density. H the mass is unequal in the two columns, isostatic com- 

 pensation is incomplete. At any plane, as x, above the level of 

 compensation, the pressure of the two columns will not be the 

 same, since the mass above this point differs. The mass of column 

 A will be greater than that of column B, and hence the pressure of 

 A at X will be greater than that of B. If now, through erosion, 

 material from the higher column A is transferred to the lower col- 

 umn B, the height of the two columns will be changed, and hence 

 the pressure at their bases will not be the same, but greater in B 

 than in A, So long as A remains higher than B, any plain, as y, 

 cutting A and B near the top, will leave A heavier than B. If, 

 then, the weight of B becomes greater than A at the base, owing to 

 the loading of B, while at y it is still less than A, owing to its lesser 

 mass, it follows that at some intermediate point, as at x, it will be 

 uniform. This is the "neutral level," which, however, rises as the 

 load on B increases and as A is lowered by erosion. Below the 

 neutral level x there will be an excess of pressure in the column 

 B over that of column A, and this excess of pressure will increase 

 as the neutral level rises, through continued erosion and deposition. 

 When the pressure becomes greater than the natural resistance of 

 the material can balance, a transference or flow of the material from 

 B to A will take place below the neutral level. This transfer of 

 material will be accompanied by an elevation of the upper part and 

 surface of A, and a sinking of the upper part and surface of B, 

 unless there is a compensatory change in volume of material. 

 Chemical changes in the mass relieved by erosion may cause further 

 expansion in volume, and consequent further rise of the surface, but 

 lowering of temperature throughout the entire block, due to the 

 lowering of the surface by erosion and the invasion of surface tem- 

 peratures into regions originally below the surface and therefore 

 of much higher temperatures, will cause a slow, but continued 

 shrinking of the mass. Hayford assumes an approximate shrink- 

 ing from this cause of the crustal column of 76 miles, to the amount 

 of 30 feet for every 1,000 feet eroded (1^:204). In like man- 

 ner, blanketing of the mass by deposition will cause a rise in tem- 

 perature and consequent expansion and increase in volume. If the 

 changes due to variations in the temperature overbalance those due 

 to the causes with opposite effects, as may be the case in the course 

 of a long time, the regions of erosion may subside, as in the event 

 of the submergence of a peneplain, while regions of former deposi- 



