THE OOLITES. 141 



texture, which the Portland limestone shows in 

 the south of England, has caused it to be asso- 

 ciated with the older Coralline Oolite, and Lower 

 . u yet marks the beginning of a great up- 

 rising of the sea-bed, which extended eastward 

 from the Mendip Hills during the succeeding 

 periods o( time. Many of its fossils may justify 



its position in the Oolitic series, but the physical 



ditions of upheaval, and representation by 



limes md sand, ap] so to link it with the 



it terrestrial epoch of l'urbcck and Wealden 

 bed-. 1 form a portion of the Neocomian 



period of time. 



The Portland strata arc almost limited in a 



ignisable form to the south of England, 

 though the horizon is defined by fossils in the 



clay beds at the top of the Kimeridge clay in 



v Hay. A thin sandy layer with characteristic 



tland fossils caps the Kimeridge clay through 

 Lincolnshire, Norfolk, and Cambridgeshire. It is 



not always possible to draw a line between this 

 esentative Of the Portland sand and 



the OVerlyipg sand termed Neocomian, so that the 



tland sand appears like the beginning of the 



shore Conditions, which lasted in the area of South 

 Britain till the close of the Lower (ireensand. 



On the Yorkshire coast there is no break 

 whatever in mineral character from the Kimer- 

 idge clay to the Hunstanton Red Limestone at 

 the base of the chalk. 



Among the fossils of the Portland beds several 

 species of Pema y Astarte % etc., are found which re- 

 semble fossils of the Kimeridge clay, but there 

 are also species of Cypruia, Pecten, Cerithium, etc., 

 which resemble Neocomian forms. Luci?ia Port- 

 landica is one of the characteristic bivalve fossils. 



