Gh. XVIII.] ATHERFIELD SECTION, ISLE OF WIGHT. 257 



No. 1. Sand, white, yellowish, or ferruginous, with concretions 



of limestone and chert .... 70 feet. 



2. Sand with green matter - - - - 70 to 100 feet. 



3. Calcareous stone, called Kentish rag - - - 60 to 80 feet. 



In his detailed desciiption of the fine section displayed at Atherfield, 

 in the south 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, distant 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 di- 

 visions. 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 mineral 

 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 predominance 

 of others. But these differences of conditions being mineral, 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 representation by new 

 and different species of corresponding genera of moUusca or other beings. 

 When the forms proper to loose sand or soft clay, or a stony or cal- 

 careous bottom, or a moderate or a great depth of water, recur with 

 all the same species, the interval of time has been, geologically speak- 

 ing, small, however dense the mass of matter accumulated. But if, 

 the genera remaining the same, the species are changed, we have en- 

 tered upon a new period ; and no similarity of climate, or of geo- 

 graphical 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 passing 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 sys- 

 tems, a break in the chain implying no doubt many missing links in 

 the series of geological monuments, 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. 296). 



* 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 va- 

 rious fossils of the lower greensand at Atherfield are given. 



17 



