204 



SCIENCE 



[N. S. Vol. XXXIII. No. 841 



adjustment, gradually increase until an 

 undertow takes place and the isostatie 

 condition is restored or nearly restored. 

 In this last state the surface of the con- 

 tinent will still be elevated, its margins 

 will still be low and the processes of ero- 

 sion, deposition and isostatie readjustment 

 by an undertow will still tend to continue. 



Note that the processes just indicated 

 explain the existence of defective density 

 (light material) in the continent and to 

 great depths below the surface, not by the 

 supposition that the light material was 

 there originally, but by the supposition 

 that the processes of chemical change are 

 such as to increase the volume and de- 

 crease the density of the material after it 

 is in position as a part of a continent. 



In studies of the earth it is frequently 

 assumed tacitly that the material is sen- 

 sibly incompressible under changes of 

 pressure produced by the shifting of loads, 

 by erosion and deposition. It would be as 

 sensible as this supposition, not more ab- 

 surd, to compare the material beneath an 

 eroded surface to the contents of a vichy 

 siphon. Upon a slight reduction in pres- 

 sure, of a few pounds per square inch, the 

 contents of a vichy siphon double their 

 volume in a few seconds. After the reduc- 

 tion of pressure caused by the erosion of 

 a layer a mile thick from the surface of 

 the earth in a given region the material 

 below to a depth of 76 miles probably 

 changes its volume by one per cent, in the 

 course of the next few ages. 



Now consider the effects of the changes 

 of temperature which would be produced 

 by the erosion, deposition and undertow 

 which have been indicated. 



Near the surface the temperature is 

 known to increase about 1° C. for each 

 100 feet increase in depth below the sur- 

 face. At great depths the rate of increase 

 is probably much smaller. Assume that it 



is 1° C. for each 200 feet on an average 

 down to the depth of compensation, 76 

 miles. Then if a stratum 1,000 feet thick 

 is eroded from a region the temperature 

 will be lowered under that region in the 

 course of ages by 5° C. upon an average 

 to the depth of 76 miles. Assuming that 

 the coefficient of vertical expansion is 1 

 part in 60,000 per degree Centigrade, the 

 material to the depth 76 miles will con- 

 tract 1 part in 12,000 in thickness or 30 

 feet. On these assumptions then for every 

 1,000 feet eroded there is a tendency to 

 produce by cooling and contraction 30 

 feet of sinking of the surface, that is, one 

 foot of sinking by thermal contraction for 

 each 33 feet of erosion. It is unimportant 

 whether this ratio 1 : 33 is a close approxi- 

 mation. It is important to note that 

 whereas the reduction of pressure caused 

 by erosion tends to make the material ex- 

 pand, the lowering of temperature caused 

 by erosion tends to make the material con- 

 tract, an opposite effect. 



Probably expansion by chemical change 

 begins to occur promptly after a certain 

 amount of erosion has occurred, since a 

 change of pressure would probably be felt 

 comparatively promptly even at consider- 

 able depths. On the other hand, the cool- 

 ing is necessarily slow and may require 

 ages to penetrate 76 miles. Hence follow- 

 ing erosion in a given region the expansion 

 due to chemical change will tend to begin 

 first. Later, and developing much more 

 slowly, the contraction due to the lowering 

 of the temperature will occur. The latter 

 may in time become as rapid or more rapid 

 than the former, the volume may cease to in- 

 crease or may even decrease, the surface 

 may stop rising or it may even sink, and 

 the region of erosion be changed into one 

 of deposition. 



Similarly, under a region of deposition 

 two effects of opposite sign tend to occur. 



