THE DOCTRINE OF ISOSTATIC COMPENSATION 717 



gravity must be forced out of sight or be interpreted by specialists 

 in the field of the exact sciences as evidence of inaccurate work. 



Becker's figures in general indicate large anomalies of gravity 

 above submerged escarpments and near where seaquakes have been felt. — 

 If it be assumed that the limit of error in Hecker's measurements is 

 not too high, his data from the 

 Atlantic (the earlier ones par- 

 ticularly) and from the Indian 

 and Pacific oceans all show a 

 maximum of anomaly above 

 the steep slopes at the margins 

 of the continental shelves, and 

 in general above submerged 

 escarpments. Examined with 

 reference to recorded seaquakes 

 the earlier data in particular are 

 hardly less interesting. Above 

 the escarpment off the mouth 

 of the Tagus, where such ex- 

 cessive values were recorded, 

 it is known that several ships 

 received heavy shocks at the 

 time of the Lisbon earthquake 

 (Fig. 4). The measurement of 

 0.146 of defect in the British 

 channel, where there is no 

 visible evidence for defect of 

 matter (this figure had nearly 

 disappeared in the earlier table 



under the mean of 0.015, and had vanished completely from the 

 later tables), is almost exactly above the spot at which a seaquake 

 has been put on record by Rudolph 1 — the only recorded one upon 

 his map within a radius of 500 miles. Other noteworthy corre- 

 spondences between seismicity and abnormal gravity will be 

 noticed upon comparing Hecker's figures with Rudolph's map. 



University of Michigan 

 January, 19 16 



1 E. Rudolph, "Uber submarine Erdbeben und Eruptionen, mit Tafel iv-vii," 

 Gerlands Beitrage zur Geophysik, I (1887), 133-365 (map at end). 



•*2 



Fig. 4. — Sketch map of a portion of the 

 Iberian Peninsula to indicate the great 

 lineament of the Tagus, along which shocks 

 were especially heavy at the time of the 

 Lisbon earthquake of 1755. The positions 

 of ships which felt the shock of the Lisbon 

 earthquake are indicated, and also the two 

 positions (crosses) where Hecker's measure- 

 ments showed excessive values. 



