GULLIVER. — SHORELINE TOPOGRAPHY. 175 



Wave-cut Islands. — On the Marblehead coast of Massachusetts we see the more 

 rapid erosion of the trap dikes which intersect the more resistant granite. 



Sir Charles Lyell has given instances of differential marine erosion in the drongs 

 of the Shetland islands.* The granite and other harder rocks longer resist the 

 waves than the schists. The many veins of porpliyry in Hillswick Ness, Lyell 

 shows, will also in time similarly be etched. 



The Orkneys and Shetlands are exposed to violent sea action, and since the 

 shore evolution is here considerably below grade, this is the place where differential 

 abrasion might be expected. The best maps of these islands t show many outlying 

 islets, high stacks, and low skerries, many of which are probably due to abrasion 

 of the sea since the drowning of this region. 



Wave-cut islands are typically seen along the west coast of Ireland. Probable 

 occurrences are in the following areas (Ireland, 9, 51, 83, 93, 103, 160, 171, 204). 



In the Southern rapids of Peril straits, Alaska (C. S., 8259), the sea is now ac- 

 tively eroding. The current, according to the Coast Survey, is often running ten 

 knots an hour, and the tides between Pinta head and Eureka ledge run with 

 terrific velocity. All the conditions are here favorable for the production of 

 wave-cut islands, and an examination of the charts shows many islands, rocks, and 

 ledges entirely isolated from each other. Tlie sea has here made no attempt to 

 simplify the irregular shoreline by connecting bars. 



"Approached by sea, the Aleutian islands seem gloom}' and inhospitable. . . . 

 An angry surf vibrates to and fro amid outstanding pinnacles." J 



Off cape Tsciiipnuski, Kamchatka, numerous rocky islets, stacks, and skerries 

 are seen upon the map, and in the sketch of Lieutenant Rogers (II. O., 54). 



At Blanca and Concon points (H. O., 12.32), and at Guacache, Cobija, and Gua- 

 silla points (H. O., 1181) on the coast of Chile. 



Algodonales point, west of Tocopilla, Chile (H. 0., 12G5). 



Submarine Platform. — The late old-age of shore development, where 

 the laud has stood approximately at the same elevation for a period of 

 time sufficiently long for the sea to have carried out its intention, is the 

 submarine platform, the plain of marine denudation. This plain will not lie 

 as far below the surface of the sea as it did in its maturity. The broader 

 expanse of the submarine platform beneath the ocean will prevent the sea 

 from so actively attacking the coast. From birth to maturity the sea 

 pushes its zone of maximum action farther and farther inland, while fiom 

 maturity to old-age the atmospheric agencies will supply more waste than 

 the shore currents can take care of, and the offshore depth will gradually 

 decrease, though the shoreline will move landward at a lessening rate. 

 The steep cliffs of maturity will diminish in height as old-age cornea 



* See figures, Principles of Geology, 11th ed., 510, 511. 



t Ivoy. Scot. Geog. Soc, Atlas of Scotland, Edinburgh, 1895, sections XLII, 

 XLIII, XLIV, XLV. 



t W. H. Dall, Sci., 1896, III. 44. 



