Uniformity in Geological Change 



of land, which will presently be spoken of, as 

 prevailing over large tracts of the globe. By 

 such elevation or subsidence certain spaces are 

 gradually submerged, or made gradually to 

 emerge: in the one case sedimentary deposition 

 may be suddenly renewed after having been sus- 

 pended for one or more geological periods, in 

 the other as suddenly made to cease after having 

 continued for ages. 



If the deposition be renewed after a long inter- 

 val, the new strata will usually differ greatly 

 from the sedimentary rocks previously formed 

 in the same place, and especially if the older 

 rocks have suffered derangement, which implies 

 a change in the physical geography of the district 

 since the previous conveyance of sediment to 

 the same spot. It may happen, however, that, 

 even where the two groups, the superior and the 

 inferior, are horizontal and conformable to each 

 other, they may still differ entirely in mineral 

 character, because, since the origin of the older 

 formation, the geography of some distant country 

 has been altered. In that country rocks before 

 concealed may have become exposed by denuda- 

 tion; volcanoes may have burst out and covered 

 the surface with scoriae and lava; or new lakes, 

 intercepting the sediment previously conveyed 

 from the upper country, may have been formed 

 by subsidence; and other fluctuations may have 

 occurred, by which the materials brought down 

 from thence by rivers to the sea have acquired 

 a distinct mineral character. 

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