670 



SCIENCE. 



[N. S. Vol. III. No. 67. 



their combinations with hmar forces are easily 

 calculated to a sufficient degree of detail. 



Although the forces available for the defor- 

 mation of the ocean are so small that the student 

 may at first dovibt their sufficiency as a cause of 

 the observed tides, his doubts vanish when the 

 consequences of the theory are systematically 

 confronted with the generalized results of obser- 

 vation, and the extraordinary 'agreements of 

 the two are discovered. Although a fairly com- 

 plete record of facts may be made by the aver- 

 age college student in the early laboratory exer- 

 cises, it is nearly always the case that some 

 classes of facts will escape his first scrutiny of 

 the tidal curves and will be revealed only when 

 attention is called to them by the expectations 

 of theory. Due attention is thus paid to the 

 difl^erent kinds of verification of theory. The 

 final acceptance of the theory becomes a logical 

 necessity, independent of the will, even though 

 certain features of the tides, especially of the 

 Atlantic tides, remain beyond the reach of the 

 elementary discvission here attempted. 



The treatment of the open-ocean tide and the 

 onshore tide, as comparable to offshore swell 

 and on-shore surf, suffices to explain various 

 facts as to age and range; and the treatment of 

 the on-shore tide as a wave accounts for the pecu- 

 liar relations often observed between flood and 

 ebb currents and high and low water. It is on 

 the basis of work of this kind that the claim is 

 made of the essentially scientific quality of 

 physiography. Although other divisions of the 

 subject may not be dealt with mathematically, 

 they all contain the logically successive phases 

 of observed and generalized facts, postulated 

 general principles, provisional hypotheses, con- 

 sequences or expectations deduced from the 

 hypotheses, comparison of the consequences 

 with the facts, and final evaluation of the knowl- 

 edge gained. Lunar gravity is the main force 

 causing the tidal changes of the sea; terrestrial 

 gravity is the main force causing the slower 

 physiographic changes of the land. 



Tidal Scour : By F. P. GULLIVER. 



The speaker considered the forms produced by 

 the tides upon flat coasts and pointed out that it 

 is wholly a question of ratios that determines 

 ±he form in any given locality. He did not agree 



with Mr. Shelford that deltas are produced only 

 in tideless seas,* for there are weak tides even 

 in the Gulf of Mexico, where the Mississippi 

 mouths, and in the Mediterranean, where the 

 Nile and Tiber deltas are found, while the 

 Ganges produces its delta in the face of seven- 

 teen-foot tides. If the river is relatively stronger 

 than the tides and other sea forces it will build 

 forward a delta. 



It is also largely a question of ratios between 

 the on- and offshore action and the alongshore 

 action which determines the production of 

 broken or continuous shore lines. Where there 

 is a broad area of marshes and flats, upon which 

 the water lies at high tide, and then during the 

 ebb scours runways beneath the level of the 

 flats, it is inferred that the tidal action is the 

 process which determines the shore forms. Off" 

 steeper coasts less tidal action is indicated. 

 Where the shoreline is prevailingly longitudinal 

 a ratio in favor of alongshore action is inferred. 



A graded series of shore forms was shown, 

 from that in which the pure tidal on- and off- 

 shore action is indicated to that in which the 

 alongshore action seems to be dominant. The 

 type of the tidal action was on the west coast 

 of Florida, where the tides are weak, but indi- 

 cations of alongshore action are absent, there- 

 fore the I'atio is greatly in favor of the tides. 

 The runways are of the indefinite consequent 

 or autogentic type of drainage, and the shore- 

 line is minutely irregular without deep indenta- 

 tions. The salt marsh grades into the tidal 

 flat. 



The type of the dominant alongshore action 

 was taken from the Texas coast. An off'shore 

 bar here forms a long gently swinging curve ex- 

 tending for 102 miles unbroken by a single tidal 

 inlet. This bar appears to have an outline 

 dominated by alongshore action. 



Along the coasts of the world various com- 

 binations of different absolute values of these 

 two actions maj' be seen in varying ratio. 

 Where the values are larger the forms have 

 greater vertical measure, as in South Carolina 

 and in the Schleswig-Holstein region. The fol- 

 lowing series of maps was shown, illustrating 

 the progressive change in ratios between the 



*Min. Proc. Inst. Civ. Engin., LXXXIL, 1885, 

 2-68. 



