proceedings: geological society 445 



applied in the elucidation of the phenomena. After referring to the 

 important work of American geologists, geodesists, mathematicians, 

 and physicists in contributing to our knowledge of the fundamentals 

 of geology, he introduced the speakers of the evening. 



Joseph Barrell, Factors in strand line movement and their results 

 during Pleistocene and post-Pleistocene. (Published in brief in Journ. 

 Wash. Acad. Sci., 5: 413^20. 1915.) 



W. J. Humphreys, Changes of sea-level due to changes of ocean volume. 

 Obviously anything that changes the amount of water in the oceans 

 also changes the sea-level at all parts of the world. Hence to this 

 extent changes in the positions of strand lines are not referable to dif- 

 ferential crustal movements. It will be interesting, therefore, to con- 

 sider how greatly and by what means the amount of water in the ocean 

 probably has varied in recent geologic times. There have been at 

 least three factors effecting such changes: (a) Annual accumulation 

 and melting of snows, — negligible in its effect on sea-level; (6) change 

 in vapor content of the atmosphere due to differences in world tempera- 

 tures—also negligible in its effect on sea-level; (c) epochal changes in 

 the amount of glaciation — effect on sea-level probably very important. 

 It will be convenient to consider these factors separately : 



a. — During northern winters snow accumulates to a greater or less 

 extent over, roughly, one-tenth of the earth's surface, or over all land 

 areas beyond latitude 40° N. Between this latitude and 60° N. the 

 annual precipitation averages about 500 mm., the largest portion occur- 

 ring during "summer. At higher latitudes the precipitation appears to 

 be less. A snow accumulation, therefore, equivalent to 50 mm. of 

 water over all this area would seem to be a conservative estimate. If 

 this estimate is approximately correct, it follows, allowing for the 

 5 to 12 ratio of land to water, that northern snows lower the sea-level 

 some 7 mm., an amount which, geologically, is negligible. This 

 assumes, though it is only approximately true, that the northern and 

 southern hemispheres compensate each other in their temperature 

 effects on the volume of the oceans and on the moisture content of the 

 atmosphere. 



b. — If we assume that the average surface temperature of the world 

 is 5°C. warmer now than it was at the time of the last maximum glaci- 

 ation, it follows that the atmosphere must now hold more water vapor 

 than then to the extent of a layer of water about 7 mm. deep. This 

 would imply that from this effect alone the ocean level is about 10 mm. 

 lower than it otherwise would be. On the other hand, the correspond- 

 ing temperature change of the oceans themselves has produced an 

 opposite effect, probably several (10, perhaps) times as great. 



c. — The fact that the average thickness bf the ice-cap during the 

 last glaciation can be only roughly estimated renders any calculation 

 of its effect on ocean level correspondingly doubtful. It does not 

 seem probable, however, that they could have averaged much if any 

 thicker than the present caps of Greenland and of Antarctica, which 



