706 



SCIENCE 



[N. S. Vol. XXXIX. No. 1011 



It is impossible that the same materials 

 eould have densities differing by six parts 

 in seventy, or nine per cent., due simply 

 to unequal contraction . from loss of 

 heat. It is conceivable that the chemical 

 elements of the earth's crust were the 

 same everywhere before solidification, but 

 they must have formed different chemical 

 compounds in order to have had such 

 different densities as must obtain in order 

 to have the earth's crust in a state of 

 equilibrium. The materials in the crust 

 below the depth of 70 miles are assumed in 

 the investigations to be in hydrostatic 

 equilibrium and consequently they should 

 have the same density in any layer at a 

 given depth. 



A theory, held by some, would make the 

 depth of compensation a function of the 

 elevation of the surface. The mountains 

 would, according to this theory, have long 

 roots of low density extending far down 

 into the plastic materials, which are sup- 

 posed to be below the seventy-mUe depth. 

 The investigations of the Coast and Geo- 

 detic Survey do not seem to substantiate 

 this theory. If it were true we should 

 have all of the crust under the continent 

 of the same density down to a certain, un- 

 known depth at which the plastic substrata 

 began. The compensation would take place 

 below that depth and it would consist in 

 protuberances extending down into the 

 plastic matter. The interior of the earth 

 must be solid, but yielding to long-con- 

 tinued stress differences. Just how the de- 

 pression of the sea bottoms is to be ac- 

 counted for under this theory, the writer 

 does not recall having heard. The crust 

 there would probably have to be very thin. 



A short article by Dr. P. G. Nutting,^^ 

 suggested to the writer a possible cause of 



i5"lsostasy. Oceanic Precipitation and the For- 

 mation of Mountain Systems," Science, October 

 6, 1911. 



part of the elevation of the continents and 

 the depression of the ocean beds. Let us 

 assume that the surface of the earth, for- 

 merly at a high temperature, had cooled to 

 a point below that of boiling water and 

 that the surface was not level. Then the 

 aqueous vapor would condense and fall in 

 great quantities as rain and remain on the 

 earth and flow to the lowest areas. This 

 added mass would tend to produce an isos- 

 tatic adjustment which would cause an 

 undertow of material from the lower areas 

 to the more elevated ones, with a resulting 

 further depression of the low areas and 

 elevation of the higher ones. How much 

 effect this could have had is diiScult or 

 impossible to estimate. Of course, if there 

 had been no irregularities in the earth's 

 surface before the precipitation, the water 

 would have covered the earth to a uniform 

 depth. 



The correct answer to another question 

 would be of great value to several branches 

 of science. The earth's crust, at least that 

 part of it within the United States, may 

 now be considered in a state of perfect 

 isostatic equilibrium. But what was its 

 condition in past geologic ages? It seems 

 reasonable to suppose that the earth's crust 

 was in the past in a state of isostatic equi- 

 librium at least as perfect as at present, if 

 it is assumed that the earth 's crust is grad- 

 ually getting cooler. With a high tempera- 

 ture the materials of the crust would have 

 been more plastic than at present and 

 should have yielded more readily to stress 

 differences. 



If we have now and always have had a 

 condition of equilibrium in the earth's 

 crust, can the recorded movements be ex- 

 plained on the isostatic theory? 



The deflections and the pendulum obser- 

 vations show that the areas from which 

 great masses of materials have been eroded 

 are in equilibrium; also that areas within 



