126 bulletin: museum of compakatiye zoology. 



" When the hxnd is sinking . . . the drowned river valleys make the 

 coast irregular and complicated." 



Another characteristic of recently sunk coasts is that fringing islands 

 usually abound along them. Immediately following a depression sedi- 

 ments brought down by the streams and washed by the waves from the 

 promontories and fringing islands silt up the bays and lower stream 

 channels, and only such channels are left as the tides are able by their 

 scouring action to keep open. At first the tide flows farther inland and 

 there ai'e many harbors on the new coast, but in the course of time the 

 silting up of the estuaries crowds the tide back to the open coast-line. 

 If the depression goes on more rapidly than silting, the tides extend 

 further and further inland ; bat if silting and the constructive work of 

 the sea are more rapid the tides are kept back. Depression brings 

 within the reach of the waves a new surface and adds gi-eatly to their 

 cutting power, and consequently large quantities of silts are produced. 

 If the prevailing winds blow on shore, as they do on the Brazilian coast 

 all the year round, the heavier silts cut from the headlands are thrown 

 back into the baj's rapidly, the beaches of sand extend seawards, the 

 closing of the mouths of streams and estuaries are hastened, and there 

 is a general tendency to form a straight shore.'^ 



The marshes built out in the depressions are soon covei'ed with man- 

 groves, and the streams in some cases Avander behind the beaches for 

 some distance before breaking through them to enter the sea. AVe thus 

 eventually have the coast marked by a succession of tide marshes and 

 lakes, with long straight beaches in front of them, and with here and 

 there a headland coming down to the sea. 



If, however, the silts cut from the shore-line are swept away by the 

 currents, either along the shoi-es or seawards, the coast soon exhibits a 

 continuous bluff and the sea encroaches upon the land until an equilib- 



1 The question might be raised as to whether the materials cut from a conti- 

 nental shoulder would be thrown on sliore or carried seaward by the undertow. I 

 do not think a fixed rule for this can be given. In some cases one thing happens, 

 and in others the otlier. In some localities the fine black silts containing marine 

 organisms are constantly thrown back into the estuaries, in other cases the coast is 

 kept clear of fine silts by the undertow. 



(The estuaries of the Severn, etc. By W. J. Sollas. Quar. Journ. Geol. Soc, 

 1883, XXXIX., p. 619; discussion by Mr. Whitaker, p. 625-626.) 



Postscriptum. — In a late article Mr. Fenneman points out that waves of transla- 

 tion tend to carry the materials on the shores upon which the waves break, while 

 waves of oscillation move materials in the direction opposite to that in which the 

 waves move. (Journ. Geol., Jan.-Feb., 1902, X., p. 13-14.) 



