I io NEWER BRITISH LIMESTONES. 



rock has the beds consisting of large and small crystals with occasional 

 altered shells of Foraminifera and Entomostraca. It is hence evident 

 that the change has been brought about by infiltrating waters. 



Lias. Lias limestones chiefly consist of joints of Pentacrinus, 

 often with fragments of brachiopods, oysters, and shell prisms, with 

 some Foraininif era and fibres of belemnites. Finer beds are almost free 

 from Pentacrinites, and are made up of fine shell sand and decayed 

 shells. Grains of glauconite occasionally occur. 



Oolites. A large proportion of the oolitic limestones consist of 

 fragments of echinoderms, oysters, brachiopods, and shell prisms ; 

 and, as compared with older rocks, contain an unusual amount of com- 

 minuted aragonite organisms mixed with rounded oolitic pellets, 

 formed by chemical precipitation round nuclei. The ferruginous 

 oolitic grains of Dundry have an unusually concentric structure. 

 They were often broken during their formation, and the fragments 

 became the nuclei for fresh grains. The fissile condition of the 

 Stonesfield slate is to a large extent due to small and thin laminae 

 derived from the shells of oysters and brachiopods. In many cases 

 the grains in the Great Oolite show a structure which suggests that 

 they were originally formed of aragonite in concentric layers, and 

 were afterwards changed into calcite. 



The Forest Marble consists to an unusual extent of fragments of 

 Terebratulae, though larger parts of the rocks are made up of echino- 

 derm fragments. At Bath it is chiefly comminuted oysters, and at 

 Frome triturated corals and polyzoa. In some beds oolitic grains are 

 abundant. The Cornbrash in Yorkshire sometimes includes vast 

 numbers of small flat crystals of carbonate of iron. The oolitic 

 grains of the Kelloway rock have generally recrystallised into fibrous 

 calcite. 



The Coralline Oolite is chiefly formed of broken calcite shells, 

 of echinoderms, Ostrea, Perna, Mytilus, Serpula, and brachiopods. 

 Some beds are chiefly formed of the shells of Renulina, others of 

 aragonite corals. Certain of the oolitic grains have for nuclei grains 

 of quartz. 



In the Portland Oolite, the building stone is a shell sand varying 

 in fineness, and mixed with oolitic pellets formed of fine granules 

 (due to decayed shells) cemented together in grains which have a 

 radiate structure. 



The Purbeck Limestones are sometimes shell sand, chiefly derived 

 from aragonite shells ; often they abound in entomostraca, and fre- 

 quently contain a large number of imperfect oolitic grains mixed with 

 shell mud. 



The Wealden Limestones are chiefly composed of fragments of 

 fresh-water shells with entomostraca, and occasional pieces of bone, 

 and in some beds laminae of oyster shells. 



The Kentish Rag in Kent consists of shell sand and shell mud 

 mixed with quartz sand and other impurities. Glauconite abounds. 

 Foraminifera are found sparingly, and there are some oolitic grains 

 derived without doubt from denudation of oolitic rocks. 



