Fbbbuaby 10, 1911] 



SCIENCE 



203 



where erosion is in progress, in the direc- 

 tion opposite to that of the surface trans- 

 fer of material. 



Let us suppose that the isostatic com- 

 pensation at a given stage in the earth's 

 history is practically complete for a con- 

 tinent, that the process of erosion from the 

 greater part of the continent and deposi- 

 tion around its margins is in progress, and 

 that the process of readjustment by a 

 deep undertow is in progress. These 

 processes will cause changes of pressure 

 and temperature within the earth at cer- 

 tain places. It is important to study the 

 probable effect of these changes upon the 

 condition and especially upon the density 

 of the material involved. 



At this point, in order to keep our sub- 

 ject in proper perspective, it is desirable 

 to recall that the average defect in density 

 under a continent corresponding to com- 

 plete isostatic compensation is one per 

 cent, or less, the average excess of density 

 under an ocean only about two per cent, 

 and the maximum defect or excess under 

 the highest parts of the continents or under 

 the deepest parts of the ocean are but little 

 greater than three per cent. These are 

 very small differences in density. Differ- 

 ences larger than these are frequently ob- 

 served between samples supposed to be 

 alike. 



If a layer of material 1,000 feet thick is 

 eroded from one part of the earth's sur- 

 face and deposited on another part the 

 pressures must become appreciably re- 

 duced for a considerable distance below 

 the eroded region and increased below the 

 region of deposition. The heterogeneous 

 material composing the earth is continu- 

 ally undergoing chemical changes. The 

 expression chemical change is here used 

 in its widest sense, the sense in which it 

 includes the processes of solution, crystal- 

 lization and changes of state between the 



solid, liquid and gaseous forms; includes 

 the solution of gas in liquids, the solution 

 of rock ingredients in water and their re- 

 deposition as new materials different from 

 the original materials, and changes from 

 an amorphous to a crystalline state, and 

 vice versa. All these and more are con- 

 cerned in the complicated processes of 

 metamorphism. In the heterogeneous 

 mixture at any point in the earth a great 

 many changes are impending. A relief of 

 pressure at any given point tends to favor 

 such changes as are accompanied by in- 

 crease of volume and reduction of density, 

 and an increase of pressure tends to have 

 the reverse effect. Many of these sug- 

 gested chemical changes are accompanied 

 by a change of much more than three per 

 cent, in density. Changes of this nature 

 in a small part of the material in any 

 cubic mile may alter the average density 

 as much as three per cent. 



A large reduction of pressure may rea- 

 sonably be expected, by favoring certain 

 chemical changes within the earth and 

 opposing others, to bring about gradually 

 with the lapse of ages a decrease of two or 

 three per cent, in the density of the ma- 

 terial relieved of pressure. 



Under a region where erosion is in 

 progress or has recently been in progress 

 one should expect, therefore, that the 

 chemical changes guided by reduced pres- 

 sure will gradually produce increase in 

 volume and a raising the surface; and 

 conversely, under a region of deposition 

 the chemical changes guided by increased 

 pressure will gradually produce increase 

 of density, reduction of volume and a 

 lowering of the surface. The surface 

 changes will then favor more erosion and 

 more deposition in the same regions as 

 before. During this process the stresses 

 due to gravitation, tending to produce an 

 undertow and thereby an isostatic re- 



