CAUSES AFFECTING POSITION OF SHORELINES. 145 



an elevation, on tlie average, of 500 feet, tlie efifect would be the elevation 

 of the sealevel 1)}' a considerable amount— by an amount probably sufH- 

 cient to liave a distinct geographic value along its shores. 



The considerations just presented enable us to see that the changes of 

 level of shoreline at any point depend upon the alterations in the form 

 of the lithosphere, which are taking ])lace all over the earth's surface, and 

 that the equation wliicli determines the conditions at any point or time 

 is of a rather com])licated nature. 



ATTRACTIOy OF CONTINENTAL MASSES. 



In addition to the foregoing influences, there are others, though rela- 

 tivel}^ minor, which have to be taken into consideration. As has often 

 been noted, the attraction of the continental masses, or even of the great 

 mountain systems, may serve to elevate the sea to a considerable height 

 al)ove its average j^lane along particular shores. Thus the water at the 

 head of the bay of Bengal is much above the level which it has on the 

 shores of the islands in the middle of the Pacific. Thus, svith the growth 

 of a great mountain-mass on the margin of a continent, the sea may be 

 drawn upward to an extent which ma}^ have measurable geographic effects. 

 It would be possible in this way to effect a difierence in heiglits, which, 

 in the case of isthmuses, as at Darien or Suez, would overdow the low 

 lying barrier. 



GLACIAL A CC UM ULA TIONS. 



Yet, further, we note that the accumulation during glacial epochs of 

 great ice-masses about either pole would in two Avays serve to change the 

 ocean })lane — by the withdrawal of water from the ocean and b}^ the 

 irregular attractions of a gravitative nature which the masses would exert 

 on the seas. If, as J. Adehemar has ])ointed out, the ice-cap were accumu- 

 lated about one i)ole to the thickness which is sometimes estimated as 

 having occurred in the last glacial i)eriod, the displacement of the earth's 

 center of gravity might amount to half a mile or more. Although this 

 estimate has little value in a (piantitative way, it is evident that ice-cap 

 displacement must be reckoned as among the considerable influences 

 wliich from time to time affect the equation of causes which determine 

 the level of tlie sea. 



Shorelines. 



general observations concerning tiiem. 



The foregoing considerations serve to prepare the observer for the study 

 of shorelines by showing him how many are the influences which serve 

 to make varia))le the vertical station of the sea. A very little observation 



