30G THE REVOLUTIONS OF THE CRUST OF THE EARTH. 



Xotliing resists the slow and continual action of marine waters, and 

 whenever they do not create they inevitably destroy. The island of 

 Helgoland, in the tenth century large and flourishing, is today reduced 

 to a mere fraction of its former size, and at every tempest loses- a con- 

 stituent part of its frail existence. The destructive force of the sea is 

 especially manifested upon abrupt projections, where the continuous 

 shock of the waves is re-enforced by the action of the tides. In the 

 Mediterranean, where the tides are nearly insensible, the action of the 

 sea is governed by the submarine currents and by the prevailing winds. 

 The oriental shores of the Adriatic are for the most part corroded, ab- 

 rupt, and deeply indented, and the number of islands is very great. 

 On the western shore, on the contrary, the sand beach is constantly 

 increased, both by the sediment of the numerous rivers, haviug their 

 source in the Alps, and by the detritus deposited at the bottom of the 

 sea. This explains why Eavenna, which at the time of the Romans was 

 a sea-port, is, at the present day, several leagues from the Adriatic. A 

 similar phenomenon is manifested upon the coast of Western Flanders, 

 where several villages, maritime in the middle ages, are now at a con- 

 siderable distance from the sea, while other towns, now ports, can only 

 be preserved by great labor from a similar fate. 



At the bottom of the sea the deposit of sedimentary rocks is formed 

 according to the nature of the material transported by the rivers or 

 washed away by the action of the waves. An estimation somewhat 

 uncertain in the present state of science gives to the sedimentary crust 

 a mean thickness of 500 meters (1,G00 feet) for the entire surface of our 

 planet. We may, however, safely admit that this thickness exceeds a 

 hundred meters (328 feet). Now as the sedimentation at the bottom of 

 one of the most corrosive arms of the ocean, the British Channel, is only 

 about four centimeters (about an inch and a half) a century,* we see 

 that the process of sedimentation even iu^ the ocean must be excessively 

 slow, and require millions of centuries to form beds equal to the vast 

 deposits of sandstone and calcareous matter. Is not the same slowness 

 of action often observed in mountain regions, where a shallow sheet of 

 water excavates a ravine of over a hundred yards in depth, while the 

 hard rock is scarcely affected by a force much greater, but not persist- 

 ent? It is not the momentary forces which produce the greatest 

 changes at the surface and in the interior of our planet; it is the smaller 

 but constant forces which, multiplied by an indefinite time, give those 

 prodigious results which dazzle our senses, and of which we fail to dis- 

 cover the origin, because we cannot conceive of the time required to 

 produce them. 



The calcareous earths, and particularly the chalk, are i:ot sediments 

 of inorganic material, but rather deposits of organized debris, formed, 

 according to Ehrenberg and several other micrographers, of the calcare- 

 ous carapaces of microscopical animals. Great masses of the pure 



* Houzeau, Z/is/oirtidw Solde L'J^urope, Bruxelles, 1857, p. 135. 



