42 THE FLOOR OF THE OCEAN 



Finland. The measurements have just recently been reported, 

 by Dr. R. A. Hirvonen. The observed values were "reduced" 

 to the geoid or sealevel, and these new values were subtracted 

 from those respectively given by the best available figure of the 

 earth. The differences are called technically "free-air anomalies 

 of gravity." To show their meaning here, in a comparatively 

 simple way, their averages for three different belts, each about 

 250 kilometers wide, were computed. The resulting averages, 

 expressed in milligals, are stated in Figure 21. Belt I, adjoining 

 the center of the glaciated tract, has 22 stations; their mean 

 anomaly is — 28 milligals. Belt II, with 62 stations, has a mean 

 anomaly of — 17 milligals. Belt III, with 77 stations, has a 

 mean anomaly of — 12 milligals. 



Thus all three belts have, in average, negative anomaly, 

 showing that there is deficiency of mass throughout Finland. 

 It is highly probable that, when the rest of the glaciated region 

 has been similarly studied, it also will show net deficiency. We 

 are particularly interested in the central area. If there the 

 mean anomaly is as much as — 28 milligals, we can conclude 

 that the defect of mass at the center is equal to that of a column 

 of granite about 250 meters or nearly 800 feet in height. 



We have arrived at two different estimates of the amount 

 of matter that is still lacking in the central area before Fenno- 

 scandia can come to equilibrium. Admittedly the data need to 

 be supplemented by more field-work; yet one may feel some 

 confidence in the belief that our results are not wildly wrong. 

 The fact that the two estimates of the defect of mass under the 

 Gulf of Bothnia rather closely agree is significant. Let us ac- 

 cept their mutual corroboration and ask: what must be the 

 order of stress under this central area, at the depth of, say, 50 

 miles ? An engineer's analysis gives the answer. By this analysis 

 the maximum stress at a depth of 50 miles is only a few atmos- 

 pheres. The stress will grow less and less as the centuries roll 



