290 IMPERFECTION OF THE Chap. IX. 



and great changes in the mineralogical composition of 

 consecutive formations, generally implying great changes 

 in the geography of the surrounding lands, whence the 

 sediment has been derived, accords with the belief of 

 vast intervals of time having elapsed between each for- 

 mation. 



But we can, I think, see why the geological forma- 

 tions of each region are almost invariably inter- 

 mittent ; that is, have not followed each other in close 

 sequence. Scarcely any fact struck me more when 

 examining many hundred miles of the South American 

 coasts, which have been upraised several hundred feet 

 within the recent period, than the absence of any recent 

 deposits sufficiently extensive to last for even a short 

 geological period. Along the whole west coast, which 

 is inhabited by a peculiar marine fauna, tertiary beds 

 are so scantily developed, that no record of several suc- 

 cessive and peculiar marine faunas will probably be 

 preserved to a distant age. A little reflection will ex- 

 plain why along the rising coast of the western side of 

 South America, no extensive formations with recent or 

 tertiary remains can anywhere be found, though the 

 supply of sediment must for ages have been great, from 

 the enormous degradation of the coast-rocks and from 

 muddy streams entering the sea. The explanation, no 

 doubt, is, that the littoral and sub-littoral deposits are 

 continually worn away, as soon as they are brought up 

 by the slow and gradual rising of the land within the 

 grinding action of the coast- waves. 



We may, I think, safely conclude that sediment must 

 be accumulated in extremely thick, solid, or extensive 

 masses, in order to withstand the incessant action of 

 the waves, when first upraised and during subsequent 

 oscillations of level. Such thick and extensive accumu- 

 lations of sediment may be formed in two ways ; either, 



