CHANGES OF LEVEL 65 



oxysmal, and appear to be nearly always the result of dislocation 

 or faulting. By change of level, in the general sense, is meant the 

 gradual, steady upheaval or subsidence of land, with reference to 

 the sea, over considerable areas. Such movements are very slow, 

 and hence are apt to escape observation ; so long are the periods 

 of time involved, that there is much dispute as to the facts, and 

 still more as to the interpretation of them. 



The method by which any change in the relative position of the 

 land may be detected, is by comparison with the sea, and hence 

 such observations, so far as they refer to changes now in progress, 

 are confined to the coast. Obviously, the result would be the 

 same if the change were in the sea, and it is somewhat doubtful 

 which is to be regarded as the seat of a given oscillation. Until 

 quite lately it was always taken for granted that the sea-level is 

 constant, and that the surface of the oceans is everywhere that of 

 a true spheroid, concentric with the figure of the earth. Recent 

 exact investigations have, however, thrown much doubt upon this 

 assumption, and indicated that the sea-level may be markedly 

 different at different places. By this is not meant the temporary 

 change due to the heaping up of the waters by the wind, or by 

 unusual tides, but a real and permanent difference of average 

 level. There is reason to believe that the attraction of the con- 

 tinents, of great mountain ranges and lofty plateaus near the coast, 

 raises the water to a higher level than in the open sea, or along 

 flat, low-lying shores. 



These local differences greatly complicate the problem of deter- 

 mining just what processes are at work in the apparent elevation 

 and depression of land. For this reason some geologists have 

 proposed to avoid the terms elevation and depression, and to 

 substitute for them " positive and negative displacements of the 

 coast-line," which are non-committal with reference to the real 

 character of the movement. For the sake of convenience, it will 

 be best to retain the older and more current terms, without insist- 

 ing that in all cases the changes lie in the land rather than in 

 the sea. 



Another process which must be carefully distinguished from the 



