36 THE STORY OI THE EARTH. 



. e more durable than the [ OsitS 



in which they were contained. 



Pebble beds indicate the eeaseless action of 

 water m wasting the earth's surface, which has 



e on without intermission through all the 



periods of past time, because tides have never 

 tsed to Bow ami ebb or rivers to run. 

 1 he positions in which pebble beds ace annulate 

 have Changed, because the outli: 



: with the folding of the ioeks during past 

 a-cs ; and they frequently come back again 

 ge with variation in level of land, at I 

 intervals to be re deposited upon a shallow [ 

 bed, or shore-line in the same district. 



Occasionally the pebbles are scratched, and 

 s<>mc in the Permian rocks of \\'< lire are 



regarded as ice-worn fragments. 



The pebbles are frequently bound together by 

 a Cement, which converts a I 



stones into a compact and durable rock. The 

 most important of these cements is silica which is 

 LSionally more durable than the pebbles winch 

 it bmds together, as may be- seen in the- II 



iddingst me. Sm h r< k Its, named con- 

 Ltes, arc- also formed of pebbles bound 

 ther with a cement of oxide of iron, or of i 



I '.'inc. 



and pebble beds a:c an 



est know n | al deposits in Britain. 



• he primary rocks the) med almost 



mite, S( in-' 5, and lavas, Am 



the CS the pebbles which make up 



[uently derived 

 ibviously had a water-formed 



In the I ' the 



i . beds \\a\i- been dei \\ ed fr< >m the 



