WHAT IS TERRA FIRMa! WILLIS. 405 



tated by the motion of the ship, but with the aid of a special appa- 

 ratus which registers that motion the effect on the barometer and 

 their actual reading can be ascertained. (PI. 2.) 



Hecker took numerous observations on voyages from Lisbon to 

 Bahia, from Bremerhaven through the Mediterranean and Suez 

 Canal to Sidney, from Sidney via New Zealand, Tutuila, and tlie 

 Sandwich Islands to San Francisco, and thence back to Japan. 

 Apart from certain anomalies in volcanic districts and in the Tonga 

 Deep, which is a vigorous earthquake center, the results correspond 

 with what the theory of isostasy requires. The intensity of gravity 

 over the ocean basins is everywhere normal. That is to say, there is 

 the same mass beneath each part of the ocean surface; each such 

 mass or column is composed of two parts, water above and rock be- 

 low. The shorter the rock part, or the deeper the water, the heavier 

 or denser the rock part must be, or, putting the relation in terms of 

 isostatic balance, we may say the denser the rock the deeper the 

 hollow in the earth's surface. 



The confirmation of the isostatic law for the oceanic basins is of 

 great importance in supporting the probability of a similar balance 

 for the continents against the ocean basins and within the conti- 

 nental masses as well. 



The present state of investigation into the subject of isostasy may 

 reasonably be summed up as follows : 



It is demonstrated that the larger masses of the outer earth, above 

 a zone 120 kilometers deep, strive toward isostatic equilibrium. 

 The condition of perfect balance has been most nearly attained 

 within the ocean basins; the general balance of the continental 

 plateaus and of the broad features of relief is at present also nearly 

 perfect. If so, it is probable that the culmination of this mountain- 

 building epoch is approaching, or is past. 



Erosion is a process which destroys those elevations of the conti- 

 nental surface which appear to be essential to equilibrium, and which 

 are probably a result of the effort toward it. The balance at any 

 time is disturbed to the extent that erosion exceeds uplift. The long 

 periods when, according to geologic evidence, lands have been low 

 and featureless, have been periods of failure of equilibrium, periods 

 of stress, when the low continental masses resisted uplift by virtue 

 of rigidity. 



Isostasy and rigidity both are conditions of the earth's mass. 

 Their relative effects in the changes of stress in the earth vary with 

 the state of uplift or erosion, and it is an interesting coincidence that 

 intelligent research should investigate the condition during an epoch 

 when equilibrium is most nearly complete and rigidity least severely 

 stressed. But we may not overlook the fact that this condition is 

 but a transient one. 



