LIFE ON THE EAETH. 131 



deep with these spoils won by the sea from the land. 

 The quantity of sediment derived from cliffs, al- 

 though computed on exaggerated data, being so very 

 much less than that obtained by the universal waste 

 of the surface of the land, may be neglected in 

 future calculations. 



Keeping still the same purpose in view, the at- 

 tainment of some probable estimate of the time 

 which was consumed in the production of some de- 

 finite part, if not the whole, of our stratified depo- 

 sits, we may take as a new basis of computation the 

 rate of growth and duration of life of some races of 

 fossil plants and animals. Thus at several stages in 

 the Coal-formation of Yorkshire and Derbyshire 

 occur beds of shale some inches or one foot in thick- 

 ness which are full of Unionidse. The area over 

 which these shells in their peculiar beds can be 

 traced is sometimes as much as 40 miles long in 

 one direction, parallel to the edge of the coal-field; 

 the breadth is considerable, may be as great, indeed, 

 but can be proved for a few miles. They have not 

 been spread over this area by drifting, but by na- 

 tural distribution of the young, as we see happen in 

 the lower parts of rivers approaching the sea and 

 the beds of lakes. Modem Unionidse live some 

 years. Suppose only the period of one life, say five 

 years, to have passed during the distribution of the 



K2 



