342 ATHERFIELD SECTION, ISLE OF WIGHT. [Ch. XVIIL 



'In his detailed description of the fine section displayed at Ather- 

 field, in the sonth of the Isle of Wight, we find the limestone wholly 

 wanting ; in fact, the variations in the mineral composition of this 

 group, even in contiguous districts, is very great ; and on comparing 

 the Atherfield beds with corresponding strata at Hythe, in Kent, dis- 

 tant 95 miles, the whole series presents a most dissimilar aspect.* 



On the other hand, Professor E. Forbes has shown that when the 

 sixty-three strata at Atherfield are severally examined, the total thick- 

 ness of which he gives as 843 feet, there are some fossils which range 

 through the whole series, others which are peculiar to particular 

 divisions. As a proof that all belong chronologically to one system, 

 he states that whenever similar conditions are repeated in overlying 

 strata the same species reappear. Changes of depth, or of the min- 

 eral nature of the sea-bottom, the presence or absence of lime or of 

 peroxide of iron, the occurrence of a muddy, or a sandy, or a gravelly 

 bottom, are marked by the banishment of certain species and the pre- 

 dominance of others. But these differences of conditions being min- 

 eral, chemical, and local in their nature, have nothing to do with the 

 extinction, throughout a large area, of certain animals or plants. The 

 rule laid down by this eminent naturalist for enabling us to test the 

 arrival of a new state of things in the animate world, is the repre- 

 sentation by new and different species of corresponding genera of 

 mollusca or'other beings. When the forms proper to loose sand or 

 soft clay, or to a stony or calcareous bottom, or to a moderate oi 

 great depth of water, recur with all the same species, the interval of 

 time has been, geologically speaking, small, however dense the mass 

 of matter accumulated. But if, the genera remaining the same, the 

 species are changed, we have entered upon a new period ; and no 

 similarity of climate, or of geographical and local conditions, can 

 then recall the old species which a long series of destructive causes in 

 the animate and inanimate world has gradually annihilated. On pass- 

 ing from the Lower Greensand to the Gault, we. suddenly reach one 

 of these new epochs, scarcely any of the fossil species being common 

 to the lower and upper cretaceous systems, a break in the chain im- 

 plying no doubt many missing links in the series of geological monu- 

 ments, which we may some day be able to supply. 



One of the largest and most abundant shells in the lowest strata of 

 the Lower Greensand, as displayed in the Atherfield section, is the 

 large Perna Mulleti, of which a reduced figure is here given (fig. 330). 



In the south of England, during the accumulation of the Lower 

 Greensand above described, the bed of the sea appears to have been 

 continually sinking, from the commencement of the period when the 

 freshwater Wealden beds were submerged, to the deposition of those 

 strata on which the gault immediately reposes. 



* Dr. Fitton, Quart. Geol. Journ., vol. i. p. 1*79, ii. p. 55, and iii. p. 289, where 

 comparative sections and a valuable table showing the vertical range of the various 

 fossils of the Lower Greensand at Atherfield are given. 



