228 PRINCIPLES OF STRATIGRAPHY 



The removal of such a bar would cause a greater rise and fall of 

 the tide on the shores of the bay, and so produce the appearance of 

 subsidence of the land. Johnson (35; 37) has used this explana- 

 tion to account for many apparent indications of recent subsidence 

 along the Atlantic and other shores. 



Comparison of Tides and ]Vavcs. Tides may be regarded as 

 huge waves sweeping successively around the earth from east to 

 west, their theoretical period being a little over 12 hours and 26 

 minutes. If lines were drawn connecting the points which have 

 the same high tide at the same moment, these cotidal lijics would 

 mark the crests of the tide waves. Normally there would be two 

 such cotidal crest lines on opposite sides of the earth, and between 

 them would be the cotidal trough line. If the earth were covered 

 by a universal ocean of uniform depth, the cotidal lines would be 

 great circles, and the period of the tide waves would be exactly 12 

 hours and 26 minutes. The velocity at the equator would be equal 

 to that of the rotation of the earth, and is approached by the tides 

 of the deep and open sea. The continents, however, greatly inter- 

 fere with this movement of the tides, and this is especially the case 

 in bays or funnel seas, where we have not only an increase in the 

 height of the wave, but also a change in interval between high and 

 low water, or between the crest and trough of the wave. As the 

 bay narrows, low water occurs nearer the following than the 

 preceding high tide, the rise being more rapid than the fall. In 

 some estuaries the duration of rise is to the duration of fall as one 

 is to ten or twenty. In such cases we have the production of the 

 tidal bore already mentioned, the water rushing up the estuary as a 

 visible wall of water with a speed of ten or more miles per hour. 



The tidal currents (flood and ebb) likewise suffer a striking 

 change. In the ocean "the flood begins three hours and six minutes 

 before high water, attains its greatest velocity at high water, and 

 ceases three hours and six minutes later." (Davis-17: I2p.) Like- 

 wise, in the ebb tide, slack water occurs at mid-interval between 

 high and low tides. This is illustrated by the tides in the center of 

 the English Channel, where the current flows up the channel (toward 

 Dover) for three hours before high tide, and down the channel for 

 three hours after. This phenomenon is understood when we com- 

 pare the movement of the waters of the tidal wave with that of 

 the ordinary wave. Rise and fall of the tide is brought about by 

 the vertical component of the orbital movement of the water, while 

 the back and forward currents are due to the horizontal compo- 

 nent. The change in the latter occurs at mid-tide, which is the 

 period of slack water. 



