62 LIFE ON THE EAETH. 



Thus it appears certain that the variety of life, 

 estimated by the marine tribes existing in a given 

 period, is greater in the more recent periods; but 

 the number of individuals, or the abundance of life, 

 is not measured by the same proportions. Periods 

 of extraordinary abundance alternate in every great 

 series of strata with other periods of comparative 

 scarcity; and though sometimes this may be ex- 

 plained by the well-known fact that red peroxide of 

 iron in sedimentary strata is very unfavourable to 

 marine invertebral life, while grey protoxidated rocks 

 of the same series contain organic remains in abund- 

 ance ; and sometimes requires attention to the un- 

 equal conservative conditions, or originally unequal 

 feeding circumstances of Calcareous, Argillaceous and 

 Arenaceous sea-bottom ; still it is a very impressive 

 phenomenon in the continuously grey Cambrians 

 and Silurians, in the continuously grey Carboniferous 

 rocks, in the continuously protoxidated Oolitic strata, 

 and in the almost uniform deposits of Lias, Oxford 

 Clay, Kimmeridge Clay, Gault, London Clay and 

 Barton Clay. An illustration is subjoined from the 

 Lower Palaeozoic Strata of Britain. Fig. 3. 



