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 post-tertiary and modern deposites^ and the 
chief difference between them and the tertiary de- 
posiles 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 XIII. 
AGENTS WmCH 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- 
