34 THE STORY OF THE EARTH, 



Pebble Beds. 



Any rock which is sufficiently durable to break 

 into ompact pieces may give rise to a pebble bed 

 when the fragments arc further reduced in dimen- 



- by the action of frost, or the transporting 

 movement of a Sowing river, or the battel 

 tion of waves upon a shore or shoal. The harder 



;S are not rounded into pebbles without 

 continued rolling. The term pebbles is applied 

 to Mones more than halt an inch in diameter, 

 that they vary in size from Barcelona nutf 



Stones which are larger than these 

 termed boulders. Stones which are smaller 



en termed .^rits. A river Bowing two miles an 

 hour transports stones as large as eggs, BO that 

 pebbles may be brought by such means from 

 many kinds o( rock which are exposed in the in- 

 terior of a country. They are mixed and ac- 

 cumulated either on shores, or where the stream 

 leaves them behind Owing to its slower m< 

 ment. 



The pebble beds around shoi 

 backwards and forwards with the daily movement 

 0l the tidal water-, and the mark, when 



up by other sediments, ancient sfa 



i which existed in bygone time. Pebl 



which exist in the old geological deposits have 



bee: d from • - .tnd SChistS, from con- 



solidated <) and lava stream-, from I 



dated sandstones, and veins of q inch 



infiltrating I in the cracks 



w hi< hi has p 



■ t pebbles I d from i 



:lar SUl , which 



