On the Elevation of Beaches by Tides. 167 



opposite shore, and continue to oscillate until destroyed by the 

 friction of its bed. But if, instead of ceasing to act, the causes 

 which produce the tide were to reappear at the opposite shore of 

 the ocean, at the very moment when the reflected tide had re- 

 turned to the place of its origin ; then the second tide would 

 act in augmentation of the first, and, if this continued, tides of 

 great height might be produced for ages. The result might be, 

 that the narrow ridge dividing the adjacent oceans would be 

 broken through, and the tidal wave traverse a broader tract than 

 in the former ocean. Let us imagine the new ocean to be just 

 so much broader than the old, that the reflected tide would re- 

 turn to the origin of the tidal movement half a tide later than 

 before ; then, instead of two superimposed tides, we should 

 have a tide arising from the subtraction of one from the other. 

 The alterations of the height of the tides on shores so circum- 

 stanced, might be very small, and this might again continue for 

 ages ; thus, causing beaches to be raised at very different ele- 

 vations, without any real alteration in the level either of the sea 

 or land. 



If we consider the superposition of derivative tides, similar 

 effects might be found to result ; and it deserves inquiry, whe- 

 ther it may not be possible to account for some remarkable and 

 well-attested phenomena by such means. 



The gradual elevation, during the past century, of one por- 

 tion of the Swedish coast above the Baltic, is a recognised fact, 

 and has lately been verified by Mr Lyell.* It is not probable, 

 from the form and position of that sea, that two tides should 

 reach it distant by exactly half the interval of a tide, and thus 

 produce a very small tide ; nor is it likely that, by the gradual 

 but slow erosion of the longer channel, one tide should almost 

 imperceptibly advance upon the other ; but it becomes an in- 

 teresting question to examine whether, in other places, under 

 such peculiar circumstances, it might not be possible that a se- 

 ries of observations of the heights of tides at two distant periods 

 might give a different position for the mean level of the sea at 

 places so situated. 



If we conceive two tides to meet at any point, one of which 

 is twelve hours later than the other, the elevation of the waters 



• See Phil. Trans. 183C. 



