KIMMERIDGE CLAY. 
335 
called by the quarrymen the “Port- Fig.sii, 
land screw” (tq Fig. 311), is common ; 
the shell of the same (b) being rarely 
met with. Also Trigonia gibbosa 
(Fig. 313) and Cardium dissimile (Fig. 
314). This upper member rests on a 
dense bed of sand, called the Portland 
Sand, containing similar marine fossils, 
below which is the Kimmeridge Clay. 
In England these Upper Oolite forma¬ 
tions are almost wholly confined to the 
southern counties. But some frag¬ 
ments of them occur beneath the IsTe- 
ocomian or Speeton Clay on the coast 
of Yorkshire, containing many more 
fossils common to the Portlandian of 
the Continent than does the same 
formation in Dorsetshire. Corals are 
rare in this formation, although one species is found plenti¬ 
fully at Tisbury, Wiltshire, in the Portland Sand, converted 
into flint and chert, the original calcareous matter being re¬ 
placed by silex (Fig, 312). 
CeritMum Portlandicum 
{zznTerehra) Sow. 
a. Cast of shell known as 
“Portland screw.” h. The 
shell itself. 
Fig. 312. 
Tsastrcea oblonga^'M. Edw. and J. Haime. 
As seen on a polished slab of chert 
from the Portland Sand, Tisbury. 
Fig. 313. 
Trigonia gihhosa. ^ natural size. 
a. The hinge. 
Portland Stone, Tisbury. 
Kimmeridge Clay.—The Kimmeridge Clay consists, in great 
part, of a bituminous shale, sometimes forming an impure 
coal, several hundred feet in thickness. In some places in 
Wiltshire it much resembles peat; and the bituminous mat¬ 
ter may have been, in part at least, derived from the decom¬ 
position of vegetables. But as impressions of plants are rare 
in these shales, which contain ammonites, oysters, and other 
marine shells, with skeletons of fish and saurians, the bitumen 
