PORTLAND ROCK. 325 



The lowest part of the clay as seen near Oxford appears in the 

 Heddington pits, resting 1 on coralline oolite. About eight feet 

 above the junction is a calcareous band eight inches thick. With 

 these exceptions, and some parts more shaly than others, the clay 

 appears nearly uniform in quality. There are probably two or 

 three beds of Ostrea deltoidea, one near the base being often recog- 

 nized, even as far to the north as Yorkshire. Here is to be seen 

 in uncommon abundance, of large size, well and finely crystallized 

 and beautifully clear, selenite, lying in the clay, and formed therein 

 by processes long posterior to the deposition of the sediment, and 

 still in progress of growth in contact with shells and bones. The 

 thickness of this deposit in Dorsetshire may be taken at 600 feet ; 

 near Swindon it cannot be much less ; but in Shotover Hill it does 

 not exceed 100 feet on the western side. There is some difference 

 between the fossil contents of the lower parts, which are few, and 

 those of the upper parts, which are mpre numerous. This is the 

 home of pleiosaurus, and tttat gigantic swimmer was accompanied 

 by plesiosaurus, steneosaurus, dakosaurus, teleosaurus, and ichthyo- 

 saurus. 



PORTLAND. 



The junction of Kimmeridge clay and Portland beds is seldom 

 to be seen near Oxford, except on the north-west side of Shotover 

 Hill, where, in 1868, the following notes were written down, on 

 occasion of examining for bones of steneosaurus found in the hard 

 sandstone boulders. 



4. Yellow clay with white irregular stripes and bands. No fossils ; a few ironstone 



lumps and pebbles. 



3. Yellow sand with bands of iron nodules and pebbles ; irregular and dis- 

 continuous. It contains a white clay band. 



In this band, at the junction with the grey sand below, are large nodular 

 blocks of gritty Portland rock, with the usual fossils. In one of these 

 nodules were found the head and cervical parts of a steneosaurus. 

 2. Grey sand with undulated stripes and vertical tracts (like fissures filled) of 



yellow sand. 



I. Kimmeridge clay, used for brick^making. The sand of No. 2 is employed to 

 complete the bricks. 



The parts marked 3 and 4 seem to have slipped down hill, really belonging 

 to the iron-sand series above. In a neighbouring pit two bands of nodules 

 appear. 



