M6 N. S. SHALER — EVIDENCES AS TO CHANGE OF SEALEVEL. 



along almost any shoreline will make it evident that the position of the 

 sea is subject to continued alternations, which are more or less marked 

 in the physical features of the lithosphere above or below the water. 

 Although a good deal has been written concerning the nature of the evi- 

 dence of such changes, there is, so far as I am aware, but httle of critical 

 value, except the work of G. K. Gilbert, based upon the study of ancient 

 lake Bonneville. I propose, therefore, in the following pages briefly to 

 consider certain criteria which may be applied to the study of shorelines 

 with reference to their oscillation, exemplifying the work from a study of 

 the coastlines of North America, principally that formed by the Atlantic 

 ocean, together with some reference^ to the present condition of other 

 coastal districts. 



DEFORMING AGENCIES AFFECTING SHORELINES. 



Pluvial Erosion. — Assuming that a sea-bottom fold attains the surface 

 of the water in the manner of recent upliftings of that nature, such as are 

 afforded by the peninsula of Florida, it is evident that the first deforma- 

 tion which occurs is that which is accomplished by the action of waves, 

 combined with that of tidal and other currents. Together with this action 

 goes the work clone on the emerged part of the elevation by the rain and 

 wind. These two groups of actions — the marine and the atmospheric — 

 work to different ends and in a diverse manner. In general it may be 

 said that the atmospheric wearing tends to lower the emerged area at a 

 very variable rate, depending as it does upon the amount of rainfall, the 

 extent to which the uplifting actions prevail against the erosion and the 

 character of the materials upon which the pluvial waters operate. Given 

 a sufficient rate of elevation, whether it be that of the fold in general or 

 of mountains originating on its surface, the effect is to diversify the re- 

 liefs by creating a system of ridges and valleys. 



Marine Erosion. — The effect of the marine erosion is brought about by 

 a horizontal attack along the shores. Along that zone the energy gath- 

 ered into the waves — it may be over thousands of square miles of water — 

 can be applied to each mile of length of coast. In the case of a newly 

 emerged fold the shorelines are normally straight or with slight curves, 

 due to the irregularities of its surface, irregularities which are generally 

 slight, so that the measure of a wave and tidal wearing is likely to be 

 nearly equal on each mile in length. Thus the normal ocean erosion 

 differs in a striking way as regards its mode of action from that which is 

 brought about by atmospheric agents ; yet further differences may be 

 noted in the secondary effects of oceanic degradation. In them we find 

 that the waste from the shore tends to enter the chance embay ments 

 along the line, and by filling theni to convert what may have been origi- 



