DIRECT INVESTIGATIONS OF ISOSTASY 185 



of matter is equivalent to that of a layer of rock about 570 feet or 174 

 meters in thickness with a density of 3.67.*^ As they point out, this 

 is small as compared with a safe working load for granite — only 660 

 ])()un(ls per square incli against 1,200 for masonry — but the conditions 

 are very different. In masonry the principal joints or contacts are hori- 

 zontal. In nature joints are usually at an angle approaching 45° to the 

 horizon, while a dry stone wall with courses inclined at 45° would have 

 no sustaining power.*^ 



Discussion of Isostasy 



This long review has been written with a view to deciding what geo- 

 logical results of geodetic research we are bound to accept. That ap- 

 proximate isostasy is a reality when areas of sufficient size are considered 

 seems to me to have been fully demonstrated. As for the unit area 

 within which it may l)c takcji for granted that isostasy is complete, opin- 

 ions differ, Mr. Helmert's estimate lieing far larger than Mr. Ilayford's. 

 This question will be settled to all intents and purposes within a few 

 years, at least so far as the United States is concerned ; for fresh stations 

 are being occupied each year, and before very long gravity maps will show 

 a mosaic of intersecting lines of zero anomaly, each closed area overlying 

 a column within which isostasy is complete. ^^ At present such informa- 

 tion as I have seems to indicate areas of from one square degree to several 

 square degrees. 



Subject to a mean error, the center of inertia of the compensation lies 

 38 or 40 miles below the surface. If compensation is uniformly distrib- 

 uted, this center lies at a depth of 38 miles, while if it diminishes linearly 

 with depth it is 39 miles. 



As yet the data are inadequate to decide between various hypotheses as 

 to the distribution of density in the active shell overlying the level of 

 isostatic compensation. This does not mean that the vertical distribution 

 of density is beyond investigation. Helmert's method of finding the 

 depth of this level is more promising in tliis respect than ITayford's, be- 

 cause it implies a determination of tlic \ci'tical couiponont of local attrac- 



*i This density is adopted from Harkness. Mr. Helmert prefers 2.73, wlilch seems to 

 me nearer the truth for the surface. For the mean density down to 120 Isilometers I 

 believe a larger figure would be preferable, perhaps 2.80. 



*= In his first paper Mr. Hayford adopted a lower estimate of the average sustaining 

 power. 'I'he mode of inference was not satisfactory and was ciilled in question by Mr. 

 Helmert. 



^•''It may be well to remember that two columns, in each of which the anotualy has the 

 same sign, may slaiul in juxtjiposlticm and convi'y the impression of a unit area larger 

 than really subsists. Possibly some of the large areas pointed out by Helmert are thus 

 composite. 



