10 



DILUVIAL AND ALLUVIAL. 



Diluvial and Alluvial. — Considerable portions of the surface of 

 the ground are, in many countries, covered with thick beds of grav- 

 el, sand, or clay, and fragments of rock and loose stones, more or 

 less rounded by attrition. In some situations, these have evidently 

 been transported from a vast distance, for frequently no rock similar 

 to the fragments occurs within a hundred miles or more of the place 

 where they are deposited. They indicate the action of torrents and 

 inundations, which have swept over the face of our present conti- 

 nents. The French have given to these depositions the name of 

 terreins de transport, a name which defines them precisely, and in- 

 volves no theory ; for it comprises, both, deposits formed, sudden- 

 ly, by migluy irruptions of the ocean, and alluvial deposits, formed 

 by the gradual deposition of sediment at the mouths of I'ivers or in 

 lakes. 



The classes of rocks above enumerated have their appropriate 

 mineral productions, and, with the exception of rocks of the first and 

 fifth classes, their appropriate organic remains ; and it would be as 

 useless to search for regular beds of common coal in the primary 

 rocks, as it would be to search for metallic veins, or statuary marble, 

 in the tertiary strata. 



It has been before stated, that we cannot be absolutely certain that 

 rocks of the same class and of a similar kind in distant countries were 

 formed at the same time. This is more especially the case with 

 rocks that contain no organic remains, such as granite, porphyry, and 

 volcanic rocks, as it is only from their relative position that we can 

 obtain evidence respecting their geological antiquity. Those rocks 

 which generally serve as the foundation for the other classes are in- 

 ferred to be the most ancient. Strata in the same class, that con- 

 tain similar species of organic remains, are admitted to belong to the 

 same geological epoch, and to have been deposited under the same 

 condition of the globe ; yet admitting that certain distant strata were 

 of coeval formation, it may be proved, that portions of the same series 

 of strata have emerged from the ocean at different intervals of time, 

 and that certain parts of the present continents have become dry land 

 at very distant and remote epochs. The ji^riod when rocks or strata 

 were first deposited has no necessary connection with the period of 

 their elevation, as will be afterwards more fully stated. 



I shall proceed to elucidate the situation of the difierent classes of 

 rocks in England, by a reference to the outline map, Plate 6. 



The waving line a a a, extending from the south-west of Dorset- 

 shire to the county of Durham, forms a striking geological division of 

 England : all the land on the east of this line is composed of the up- 

 per secondary and tertiary strata, in which neither metallic veins nor 

 regular beds of mineral coal are foiind. The tertiary strata lie over 

 the upper secondary, within the parts bounded by the letters ooo o. 

 On part of the eastern coast of Yorkshire and Lincolnshire, there is 

 a submarine forest about seventeen feet under the present highwater 



