434 



SIR CHARLES LYELL 



such species as live at moderate depths, may be formed and 

 may increase in thickness to any amount. It may also ex- 

 tend 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 fossiliferous rocks will always 

 exist, and the imperfection 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 undergoing constant 

 upheaval, will usually be too slight in thickness to endure 

 for ages. 



In proportion as we become acquainted with larger geo- 

 graphical areas, many of the gaps, by which a chronological 

 table, like that given at page 135, is rendered defective, 

 will be removed. We were enabled by aid of the labours 

 of Prof. Sedgwick and Sir Roderick Murchison, to inter- 

 calate, 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 fauna of 

 these last-mentioned formations wanted the connecting 

 links which now render the passage from the one to the other 

 much less abrupt. In like manner the Upper Miocene has 

 no representative in England, but in France, Germany, and 

 Switzerland it constitutes a most instructive link between 

 the living creation and the middle of the great Tertiary 

 period. Still we must expect, for reasons before stated, that 

 chasms will for ever continue to occur, in some parts of our 

 sedimentary series. 



Concluding remarks on the consistency of the theory of 

 gradual change with the existence of great breaks in the 

 series. To return to the general argument pursued in this 

 chapter, it is assumed, for reasons above explained, that a 

 slow change of species is in simultaneous operation every- 

 where throughout the habitable surface of sea and land; 

 whereas the fossilisation of plants and animals is confined to 

 those areas where new strata are produced. These areas, as 

 we have seen, are always shifting their position, so that the 

 fossilising process, by means of which the commemoration 

 of the particular state of the organic world, at any given 



