Uniformity in Geological Change 



thickness of sedimentary matter accumulating 

 at depths suited to the habitation of most of the 

 species of shells can never be great, nor can the 

 deposits be thickly covered by superincumbent 

 matter, so as to be consolidated by pressure. 

 When they are upheaved, therefore, the waves 

 on the beach will bear down and disperse the loose 

 materials; whereas, if the bed of the sea subsides 

 slowly, a mass of strata, containing abundance 

 of such species as live at moderate depths, may be 

 formed and may increase in thickness to any 

 amount. It may also extend horizontally over a 

 broad area, as the water gradually encroaches 

 on the subsiding land. 



Hence it will follow that great violations of 

 continuity in the chronological series of fossil- 

 iferous rocks will always exist, and the imper- 

 fection of the record, though lessened, will never 

 be removed by future discoveries. For not only 

 will no deposits originate on the dry land, but 

 those formed in the sea near land, which is under- 

 going constant upheaval, will usually be too 

 slight in thickness to endure for ages. 



In proportion as we become acquainted with 

 larger geographical areas, many of the gaps, by 

 which a chronological table is rendered defective, 

 will be removed. We were enabled by aid of the 

 labours of Prof. Sedgwick and Sir Roderick 

 Murchison, to intercalate, in 1838, the marine 

 strata of the Devonian period, with their fossil 

 shells, corals, and fish, between the Silurian and 

 Carboniferous rocks. Previously the marine 

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