CHAPTER IX. 



COAST-LINES AND THEIR ORIGIN. 



EVERY part of the earth which rises out of the sea is distinguished by 

 its own peculiar outline. This outline, in which the ocean marks a 

 definite level around the land, is the sea-coast. Its fantastic curves 

 on some shores, and scarcely broken straight extent on other lands, 

 are not a matter of accident ; for the causes which raise islands from 

 the sea, also determine the main directions in which the coasts run. 

 Inlets, bays, channels, and headlands may have to be explained by dis- 

 covering the courses of old rivers, or the work of rain, and the kinds 

 of rocks exposed; but the coast line has been produced slowly at 

 successive ages of the earth's history, and parts of it have from time 

 to time been portions of lands of far different outline to those of 

 existing continents and islands, though the ancient lands are now 

 more or less destroyed and submerged. 



Influence of Altitude. iSTothing perhaps will help so well to make 

 intelligible the first and simplest law under which a coast line may 

 change, as to take a map on which are drawn lines showing the 

 course taken over the country by contours indicating levels at ever- 

 increasing heights such as would be marked by the sea, if the land 

 were submerged to that extent. Then the successive steps would be 

 traced by which a large mass of land may become broken into islands, 

 and the reason why the smaller islands are formed would be more or 

 less clear, for the sea necessarily would cover the low land first. 

 Similarly with the sea ; lines which mark depths of increasing 

 amount in hundreds of feet enable us to understand how islands may 

 be enlarged, united together, and into continents, and have the course 

 of their coast line changed, by being merely uplifted so that the sea 

 drains off from regions which it once covered. 



Wherever a coast line remains for some time unchanged in level, 

 the wearing power of the tides will usually convert what had previously 

 been a shelving shore into a sea-cliff. If, then, land is upheaved at in- 

 tervals, with periods of pause during which no upheaval takes place, then 

 inland cliffs will be formed which correspond to these intervals of rest. 

 The position in which cliffs are produced is often governed by the way 

 in which the layers of rock forming the country are arranged. This 

 arrangement of the strata into hard beds and soft beds is accompanied 

 by an inclination of the deposits technically called "dip." The sea 

 acting upon deposits so inclined abrades and wears away the exposed 



