3IO JOSEPH BARRELL 



its limits. The major relief of the continent above the ocean bot- 

 toms would be about nine-tenths eliminated and the mean eleva- 

 tion of all areas as great as several hundred miles in width would 

 be reduced to a small figure. This is the effect of isostasy. But 

 within these unit areas which measure the limits of regional com- 

 pensation would everywhere rise a rolling mountainous surface. 



Imagine the hypothetical surface as broad as the United States, 

 concealed from view by an impenetrable envelope of cloud, and 

 aerial explorers to sink a sounding line to this invisible land at 1 24 

 places chosen at random. The resulting contour map compiled 

 from these soundings would yield a much smoothed and flattened 

 surface such as is shown in the contour map of gravity anomalies. 

 Many of the soundings taken really on mountain slopes, because 

 they were the highest of those made, might be casually interpreted 

 as located on mountain peaks. The latter, standing sharp and high, 

 would be missed save for an occassional lucky chance of the sound- 

 ing line. 



Interpreted in terms of weights and stresses, it is seen that even 

 the parts of the continent appearing to the eye as plains long in 

 geologic quietude really conceal within them strains as great as 

 those imposed by the weight of mountains. That these great 

 strains have been born for geologic ages, in many locahties probably 

 from the Archeozoic, gives a surprising conception of an enduring 

 rigidity and elasticity of the crust wholly at variance with certain 

 current doctrines regarding the weakness of this zone. It is not 

 here found to be a failing structure. 



On p. 81 Hayford and Bowie give the new-method anomaHes 

 for sixteen stations not in the United States. An abstract is given 

 below of the greater anomaUes from that table with the addition 

 of Seattle. The thickness of stratum taken as corresponding to 

 the anomaly is also added. This thickness, if the compensation 

 is uniform with depth, measures the distance by which the earth's 

 surface is out of isostatic equilibrium at those points. A plus sign 

 indicates an excess of mass and a consequent tendency to sink, 

 resisted by rigidity; a minus sign a defect of mass and therefore 

 the existence of an upward strain. 



The divisor 0.003 dyne of anomaly, taken as the equivalent 



