Shaler.] 410 [Dec. 7, 



tidal waves in that region, the result would be the great and imme- 

 diate diminution in the run of the tides in that section. At the 

 close ©f the last glacial period, during the time when the land re- 

 mained depressed much below its present level, it seems certain 

 that this passage was wide open and there are reasons to believe 

 that the energy of the tides in the Bay of Fundy was much less 

 considerable than at the present time, as is shown by the fact that 

 the amount of cross-bedding exhibited in the stratified gravels at 

 heights greater than that of the land which separates these two 

 areas of water indicates a smaller amount of tidal action than is 

 indicated by the strata now forming along the shore. 



The change in the regimen of tidal currents is propagated to 

 great distances from the point where the cause of the change oc- 

 curs. Thus if Cape Cod should be cut through by the action of 

 the tides, the effect in the run of the currents along the shore would 

 be felt for many scores of miles in the direction in which the tide 

 moves. In this way we may account for very many alterations in 

 the direction of the bedding in our sandstones which have been due 

 to the action of tides. Changes in the level of the sea floor on 

 which sediments are deposited naturally induce profound differences 

 in the character of the beds which are formed upon it. This re- 

 sult is brought about in either of two ways. In the first place, by 

 bringing the point in question nearer to or removing it farther from 

 the shores, the character of the sediments derived from the land is 

 much altered. In the second place, the changes in the depth of 

 water are apt to lead to a modification in the character of the or- 

 ganic life which enters into the formation of the deposits and thus 

 produce a considerable modification in the nature of the sediments. 

 As before remarked, several students have noted the fact that many 

 cycles of change in rocks are to be explained by these alterations 

 in the depth of the sea floor and consequently in the remoteness of 

 the land. In general the process of subsidence is marked by a 

 diminution in the amount of material derived from the land and a de- 

 crease in the average size of the grains which compose it. Where 

 the alternation of level brings a deep- sea area to the condition of 

 shallow water or the reverse, the range of variation brought about 

 by the change of conditions is often very great and is shown not 

 only in the character of the sediments themselves but in the nature 

 of the organic fossils embedded in them. 



Well-known instances of a wide swing in the conditions of organic 



