132 ALLUVIAL AND DILUVIAL DEPOSITES. 



of " the flood," we think it far more philosophical 

 to attribute the phenomenon in question to such a 

 convulsion. 



The alluvial and diluvial formations have been 

 called posl-iertiary and modern deposites, and the 

 chief difference between them and the tertiary de- 

 posites is, that in diluvium the sand, pebbles, and 

 clay are confusedly mixed together; whereas, in 

 the tertiary formations, the materials are arranged 

 in regular, and generally in horizontal layers, one 

 above another. Mr. Philips remarks, that it is often 

 extremely difficult to say whether certain aggrega- 

 tions of sand, gravel, and shells are of tertiary date 

 or the productions of later times. Enormous heaps 

 of pebbles and bones lie in particular situations, and 

 are evidently of great antiquity ; but whether of the 

 tertiary era or not, requires much care in determin- 

 ing. Certain lacustrine deposites, full of shells, 

 marls, peats, and bones of stags, cannot, by a hasty 

 glance, be known from tertiary strata collected from 

 ancient lakes. But, upon farther and closer scrutiny, 

 geologists have generally agreed that a whole se- 

 ries of deposites, partly marine, partly terrestrial, 

 lacustrine, and fluviatile, has been formed since the 

 date of the truly tertiary strata. 



CHAPTER XIIL 



AGENTS WHICH DESTROY ROCKS. 



Proofs of Changes on the Earth's Surface. — Mechanical Agents 

 which Destroy Rocks : Rains, Torrents, Rivers, Seas. — The 

 Atmosphere. — Influence of the Sea upon the Land: In Eu- 

 rope — In America. 



It is quite a prevalent opinion among the common 

 people, that rocks came from the hand of the Crea- 



