16 Dr. Wi'ight on the Geology of the 



tions in the thickness of the beds in different parts of their course. 

 This fact of local activity during the deposition of these strata 

 forms an interesting feature of our English tertiary system. 



It admits of demonstration in several of the minor groups, but 

 becomes strikingly evident when we compare the section at White 

 Cliff with tiiat at Alum Bay, where the difference in thickness 

 amounts to upwards of 300 feet in the entire series. This fact 

 accounts for the difficulty experienced in making measurements 

 of the same beds tally at different points, and therefore our 

 figures must be received only as approximations to the truth. 

 During the accumulation of these strata, irregular local action 

 was going on at very short distances apart, as proved by the di- 

 versity which exists between our section and the equivalent beds 

 in the Hampshire basin, and shows how necessary it is, in the 

 study of our tertiary system, to multiply observations upon the 

 individual beds in different parts of their course, and not to 

 confine our observations to one section alone. 



The beds may be classified into — 



1. Lacustrine. 



a. Upper freshwater. 



b. Lower freshwater. 



2. Estuary. 



Intercalated with the above. 



3. Upper marine. 



4. Lower marine, divisible into — 



a. Barton series. 



b. Coloured sands and clays. 



c. Bognor series. 



The lacustrine strata contain the genera Paludina, Lymnaa, 

 Planorhis, Melania, Melanopsis, Cyclas, Potamomya, Unio. 



The estuary strata contain Potamides, Melanopsis, Melania, 

 Natica, Nerita, Neritina, Cyrena, Mytilus, Ostrea, and Sei-pulce. 



The marine strata contain Ancillaria*, Voluta*, Natica, Bulla*, 

 Murex*, Cancellaria*, Rostellaria* , Fusus, Cytheraa*, Psammo- 

 bia*, Mactra*, Mytilus, Ostrea, Balanus*, Serpulee*. 



I rarely found estuary shells mixed with the true lacustrine 

 genera, but occasionally a few Lymncece or Paludince were found 

 in an estuary bed. Cyrena and Potamides seem to have been 

 common to beds of estuary and marine origin ; the true marine 

 genera marked (*) are limited to the marine formations. 



If the premises upon which the argument is based be correct, 

 it follows that many changes of condition took place during the 

 deposition of the lacustrine series, as there are several interca- 

 lations of estuary genera between the true lacustrine beds both 

 in the upper and lower formations. 



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