Coral Rag 17 



Typically the Oolite is a creamy and not very compact lime- 

 stone, consisting of flattened or oval grains about half the size 

 of a pea. Hosts of tiny gasteropods and lamellibranchs are 

 represented by casts among the grains, and only large calcite 

 shells survive in their entirety. Gervillea, Opis Mytilus and 

 Pecten are the commonest fossils, but in certain beds, especi- 

 ally near the top of the series, sea-urchins such as Holectypus 

 dqpressus, Echinobrissus scutatus and Pygaster umbrella 

 abound. 



Above the Oolite comes the Rag, which consists chiefly 

 of recrystallized tabular coral growths. It is platy, full of 

 holes and very hard, and is in fact a typical coral reef. The 

 chief reef builders were corals of the genera Isastraea, Mont- 

 livaltia, Stylina and Thamnastraea, and these are associated 

 with very numerous lamellibranchs such as are common even 

 now on coral reefs, e.g. Opis, Area and Ltihodomus. Gastero- 

 pods also are abundant, and urchin fragments are by no 

 means rare. Cidaris florigemma is the most plentiful. 

 Ammonites are rare. 



Occasionally when Gault is being dug at the south-east 

 corner near the river entrance, another mass of Rag is to be 

 seen. It immediately underlies the Lower Greensand, and 

 must, I think, be identical with the rag and rubbly beds 

 found in the boring, as its position is such that unless some 

 unseen fault intervenes it cannot overlie the main mass of 

 Oolite. 



The total thickness of limestone exposed in the pit must 

 be about fifty feet, of which about twenty belongs to the 

 Coral Rag, and the rest to the Oolite and underlying beds. 



The North pit is about one and a half miles north of this 

 main pit : no fresh rock can now be seen there, but urchins 

 can still often be picked out among the rubble near the 

 water's edge. The beds are very similar to the middle beds 

 of the Oolite of the South pit and the faunas are identical. 

 The beds of the North pit still dip to the north at a low 

 angle, and hence must undulate considerably in the interval 



M. & S. 2 



