544 STUDIES FOR STUDENTS 



that the underground capillary rise is no longer able to keep the 

 surface wet. This time limit will vary with the climate and the 

 texture of the clay, but there may be immediately excluded all that 

 portion of the littoral which is wet twice per day; in other words, 

 all that portion of the littoral below the upper limit of the neap tides. 

 This may be modified to some extent by strong off-shore winds. In 

 the temperate zone such winds, being usually of a cyclonic nature, 

 are frequently accompanied by rain; but where not, it is possible 

 that by this means the tidal rise may be prevented from reaching its 

 normal level by some feet and mud-cracks formed in the meantime 

 somewhat below the usual level. In the latitudes of monsoon winds 

 such effects might be seasonal, as is noted in the Runn of Cutch on 

 the southeastern side of the Indus delta. Off-shore winds, therefore, 

 will permit a wider development of mud-cracks over the upper 

 portion of the littoral zone, but it is not probable than any appreciable 

 areas below the level of mid-tide should be laid bare, dried, and 

 cracked by such means. Neither has such an effect been described. 



In tideless seas the fluctuations of level due to storms are impor- 

 tant. Where there is an open reach of water, however, the waves 

 which develop upon its surface break off-shore at a depth which the 

 writer has seen stated somewhere as half the height of the wave 

 below the trough of the same. This action maintains an open sea 

 and an effective working depth, since the waves as soon as they 

 drag on the bottom scour it out and carry the material partly on to 

 the beach, partly into deeper water. In order, then, that any appre- 

 ciable stretch of bottom normally covered by water should be laid 

 bare, the change of water level between the on-shore and oft'-shore 

 storms would have to equal at least the height of the waves of the 

 on-shore storms. 



As an instance of changes of level under favorable circumstances 

 may be mentioned those of Lake Erie, a narrow body of fresh water 

 245 miles long lying in a northeast and southwest direction and 

 therefore subject to heavy gales blowing the length of the lake from 

 both directions. As a result Whittlesey has noted a change of level 

 at Buffalo of 15I feet between flood water and low water. ^ At 

 intermediate points such as Erie and Cleveland there is naturally 



I Dana, Manual of Geology, p. 202. 



