HISTORY FROM THE ROCKS 195 



first mentioned are of course of the greatest service, and it is 

 a common practice to name the bed from one or more of its 

 characteristic and exclusive fossils. The recognition in a new 

 district of one or more beds containing fossils characteristic of 

 strata in a known position elsewhere gives a clue to the age of 

 the rocks in the new district from which their position, succession, 

 and relations can be worked out. Indeed, it is to this discovery 

 by William Smith at the end of the eighteenth century that all 

 modern progress in historical geology is due ; and the early 

 pursuit of the method in Britain rendered the names given to 

 members of the British succession applicable to their equivalents 

 not in Europe only, but in many other parts of the world. Such 

 terms as Silurian, Devonian, Carboniferous are of world- wide use. 

 In collecting fossils from successive beds, sometimes the life 

 change from bed to bed is rapid and complete, but more usually 

 it is gradual and less decisive. The first condition indicates a 

 rate of deposition which is slow even compared to the rate of 

 organic change ; the second indicates more rapid deposition, 

 and convenient divisions of the strata are much less easy to 

 locate and define, and are necessarily much less natural in 

 character. The first condition is associated with deep, clear 

 water, to which the supply of sediment is brought only in very 

 small quantities. Confirmatory evidence is usually given by 

 the fineness in grain of the deposits, the relative abundance of 

 fossils, and the association with deposits such as are now known 

 to be forming slowly in the ocean. On the other hand, the more 

 rapidly deposited rocks are coarser in grain, more irregular in 

 structure, deposited in shallower water nearer to a coast-line, 

 and in proximity to regions of more rapid denudation. Thus 

 more evidence is given as to the physical condition of the area 

 during the period under consideration. As might be expected, 

 the slowly deposited rocks will be of wide geographical extension ; 

 they will be most easy to correlate from place to place ; and they 

 will form the main ties linking together the general succession 

 in one place with that in another. 



