author's previous views as to sealevel changes. 143 



briefly referred to the influence of such swaj^ngs of the crust beneath 

 the thaUassal areas. 



Conditions and Actions affecting Sealevel. 



SYXOPSrS OF A UTirOR'S PREVIOUS VIEWS. 



Some years ago I undertook to present'^ in a formal way a list of those 

 actions which are or may be concerned in the changes of level of the 

 coastline. As the points therein discussed are of importance with refer- 

 ence to the matter which is to be taken u}) in the following pages, I ven- 

 ture to recapitulate them as follows : 



Although the direct up and down movement of the land is the most 

 obvious because the simplest possible explanation of coastline changes 

 in level, it is doubtful whether it is the prevailing or even the frequent 

 mode in which tliese alterations of height are effected. Occasionally, b}^ 

 the faulting of blocks of strata or the accumulation of volcanic matter 

 below a crater, or even by the injection of dike materials in crevices which 

 do not attain the surfiice, local vertical movements probably occur, but 

 in most instances the swayings of the earth must be regarded as Avide- 

 spread phenomena connected with the tolerably steadfast process by 

 which the continents go upward while the sea-basins deepen. In other 

 words, the sections of the crust involved in the movement which affects 

 the coastline are tilting upward on the continental side and downward 

 beneath the sea. The movement is like that of a lever with a variable 

 fulcrum point — a pt)int of bearing which itself may be subjected to some 

 dislocation. In this movement about a variable node, which we must 

 conceive as taking place on the continental slopes from the centers of 

 the land areas to those of the sea, it is evident that the shoreline may 

 have a variable position. It may Ijy chance, though probably seldom, 

 occur that the contact of sea and land is just over the node, in which case 

 the rotative movement, so far as it determines the position of the coastline, 

 may be imperceptable. lUit if tlie neutral point is seaward, the effect 

 will l)e to lift the land above the water. If, however, the fulcrum be to 

 the landward of the coast, the same movement would cause the sea to 

 gain on the land. Thus, so far as the changes of coastline are concerned, 

 which are brought about by the normal downgoing of the sea-floors and 

 uprising of continental areas, three different conditions may come from 

 the same action : that of no motion of the coast, that of uprising, and tluit 

 of downsinking of the land. 



The above simple considerations show very clearly that the i)lRMU)mena 

 of elevation of continents as regards the action on shorelines is much 



♦ Proceedings Boston Soc. Nut. Hist., vol. .\ii, October 7. 



