84 



LYELL'S ELEMENTS OF GEOLOGY. 



Relation of Alluvium to regular Strata. 



Fig. 83. 



nean streamlets, which have flowed 

 through and enlarged natural 

 rents ; or, when on a small scale, 

 they may "be spaces which the 

 roots of large trees have once oc- 

 cupied, gravel and sand having 

 been introduced after their decay. 

 It is not so easy as may at first 

 appear to draw a clear line of dis- 

 tinction between the fixed rocks, or 



a. vegetable soil. regular strata, (rocks in situ^ or in 



b. alluvium. \ j i n i 



c. mass of same, apparently detach- pZace,) and their alluvial covering 



of travelled materials. If the bed 



of a torrent or river be dried up, we call the gravel, sand, and 

 mud, left in their channels, or whatever, during floods, they may 

 have scattered over the neighbouring plains, alluvium. The 

 very same materials carried into a lake or sea, where they be- 

 come sorted by water, and arranged in more distinct layers, are 

 termed regular strata. 



In the same manner we may contrast the gravel, sand, and 

 broken shells, strewed along the path of a marine current, with 

 strata formed by the discharge of similar materials, year after 

 year, into a deeper and more tranquil part of the sea. 



If any fossils occur, the mass may still be called alluvial, pro- 

 vided the fossils appear to have been drifted to the spot. If any 

 of them, as, for.gxample, freshwater or marine shells, seem to 

 have lived and died where they are entombed, then the deposit, 

 though mainly consisting of drift materials, should not be termed 

 alluvial, but a regular marine or freshwater formation. It is, 

 however, easy to perceive that passages must occur from such 

 alluvial to regular deposits, both in the sea and the estuaries of 

 rivers ; and it is often most difficult to distinguish between them, 

 because organic remains have been often obliterated in forma- 

 tions of porous sand, gravel, and loam, which allow rainwater 

 to percolate freely through them. 



After what has been said of the connexion of denudation and 

 alluvium, the student will expect to find alluviums of various 

 ages, and at all heights above the sea, formed both before and 

 during the emergence of land, but always most copiously at 

 periods when the level of a country has undergone changes by 

 subterranean movements ; for then the course of running water, 

 whether marine or fluviatile, has been most frequently deranged, 

 and the power of the waves of the ocean has been brought to 

 bear with the greatest effect against the land. 



Before tfie doctrine of the rising and sinking of large conti- 



