Physical Geography. 193 



sometimes marine, as Pholadomya, Modiola, Ostrea, 

 Necera, &c. ; at other times fresh- water shells, as Cyrena, 

 Unio, &c., and he correctly states that ' all the cha- 

 racters presented by the beds of the Upper Estuarine 

 Series, point to the conclusion that they were accumu- 

 lated under an alternation of marine and fresh-water 

 conditions, such as takes place in the estuaries of rivers.' 

 These strata between Northampton and Grrantham are 

 rarely more than about 25 feet in thickness. 



When we think of the meaning of these phenomena, 

 it is evident that, while from Gloucestershire to the south 

 coast, all the strata from the base of the Lower Lias to 

 the top of the Oolitic series are marine, in the middle 

 area of Northamptonshire, Eutland, and Lincolnshire, a 

 set of conditions prevailed in the time of the deposition 

 of the Lower Oolites that indicated filling up of the area, 

 and temporary elevation of the old marine deposits, in 

 places, quite above the level of the sea, so that swampy 

 terrestrial surfaces were formed, through which wandered 

 minor streams inhabited by fresh- water shells. Further 

 north this fact becomes still more plain. 



After crossing the Humber, and passing the uncon- 

 formable overlap of the Cretaceous rocks of the Yorkshire 

 Wolds, a series of Liassic and Oolitic strata appears in 

 the North Eiding, forming a great tract of beautiful 

 hilly country, the sections of which are best seen on the 

 coast cliffs that lie between the mouth of the Tees and 

 Filey Bay. That part of the cliffs of which the strata 

 are of Oolitic age, more or less includes representatives 

 in time of all the so-called formations from the Inferior 

 Oolite to the Kimeridge Clay inclusive. The lithological 

 characters, and mode of formation, of all the strata that 

 are presumed to lie between the horizon of the base of 

 the inferior Oolite and the Cornbrash, are, however, of a 



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