T1IK .IfltASSIC SYSTEM 



117 



Upper 

 Portlandiau 



Lower 

 Portlandian 



is not more than 30 feet thick, while the sandy be<l.- an- about 

 60 feet, so that the whole is less than 100 feet. In the Vale of 

 Wardour both chalky ami oolitic limestones occur, ami t-arh division 

 of tin- sta^c is tVoni 50 to GO feet thick. 



The next good exposure of the Portland Beds is at Swindon, 

 where the limestones are partly replaced by sands. The beds are 

 exposed in railway cuttings and quarries, and the combined 

 succession is given by Mr. H. 13. Woodward as follows : 



6 to 12 

 20 to 25 



3 to 74 

 . 14 to 20" 



6 to 8 

 . 30 to 40 



About 100 



In the Midland counties the only good exposures of the 

 Portlaiulian are in Buckinghamshire near Thame, Brill, and 

 Aylesbury, where they exhibit a special facies different from that 

 of Dorset or Wiltshire. The succession is as follows : 



Feet. 



Grey marls and thin limestones, Holcostcphamts 

 yiganteus, Ostrea cxpansa, and Trigonin 

 gibbosa . . . . . . . . 8 to 12 



Mai ly and shelly limestones, Trigonia gibbosu . 5 to 8 

 Greenish-yellow sand . . . . . 5 to 6 



Marly and rubbly limestone with fossils . . 10 to 15 

 Yellowish-green sand with a bed of small pebbles 



of quartz and lydianite at the base . . . 8 to 10 

 Fine sandy clay with Astarte hartwellensis, Car- 

 dium morinicum, Perna Boucliardi, Trigonia 

 Pellati . 



( Marly and oolitic limestones .... 

 Whitish sands with lenticular layers of cal- 

 -! eareous sandstone ..... 

 I Marly and oolitic limestones with Trigonia 

 \ gibbosa, and CerithiumportJundii-i/m . 



(Blue clay weathering brown .... 

 Marly sandstone with Exogyra bruntittana 

 Sands with doggers of calcareous sandstone . 



Upper 



Portland, 



26 feet 



Hartwell 

 Clay 



20 to 30 



Near Thaine there are yellow and green sands below the pebble- 

 bed, but these thin out or pass into clay northward ; the Hartwell 

 Clay being apparently the equivalent of the Portland Sands. 



The most northerly outlier of these beds is at StewkK-y Warren, 

 4 miles west of Leighton Buzzard, where about 15 feet of them, 

 capped by a few feet of Purbeck Beds, was at one time expo.-rd. 

 Beyond this they are not seen either in Bedford, Cambridge, 

 Norfolk, or Lincoln, but derived and phosphatised Portlandian 

 fossils are of frequent occurrence in the Lower Cretaceous sands, 

 and consequently it is most probable that they originally extended 

 through these counties, but were destroyed at the beginning of the 

 Cretaceous period. 



In Yorkshire, at Speeton, the Kimeridge Clay is surmounted 



