128 LIFE ON THE EARTH. 



a sort of delta, or along a limited range of coast- 

 line. The Coal-field of South Wales is connected by 

 the character and succession of the Strata with those 

 of the Forest of Dean, Kingswood, and Somerset- 

 shire. This gives a length of 125 miles and a maxi- 

 mum breadth of 50, which may be assumed to have 

 been one great deposit, on the southern side of one 

 line of coast. If the 6250 square miles thus cal- 

 culated were all deposited from the spoils of one 

 great river equal to the Ganges, something less than 

 half an inch would be deposited in a year, and 12,000 

 feet in 333,000 years. 



If we vary the computation by taking what may 

 be thought a fairer basis of comparison, and suppose 

 a stream equal to the great Indian river to have 

 produced the Wealden deposits 3000 square miles 

 and 1000 feet thick twelve thousand years would 

 accomplish ,he work, but the Wealden deposits pro- 

 bably extended to more than four times the area, 

 though with a less average thickness. 



But the sea also attacks the land, often very 

 strenuously, and along the vast line which it beats 

 with restless energy, many parts of the coast yield 

 much, all yield something, to the watery hammers. 

 The rate at which the sea wastes the land depends 

 on the nature of the coast, and the force and direc- 

 tion of the currents. A cliff of shale or glacial de- 



