ORGANIC REMAINS. 



SOS 



eral substances, held to be elementary by the chemist, may be re- 

 solved into original elements still more simple, and afterwards recom- 

 pounded into other substances, we have no reason to mount so high 

 in our speculations, respecting the origin of flint. 



Flint is siliceous earth nearly pure ; and we find the same earth 

 under different forms, and in a greater or less proportion, combin- 

 ed with almost all calcareous rocks. 



Primitive limestone is often much intermixed with siliceous earth. 

 Transition limestone occasionally contains rock-crystals imbedded in 

 the mass : this is, not unfrequeqUy, the case in some of the transition 

 limestones of Derbyshire. TI73 magnesian limestones and oolites, 

 are, also, very commonly intermixed with siliceous grains, and, often, 

 alternate with strata that are more or less siliceous : hence, we need 

 not be surprised to find silic/jous earth in chalk, either combined with 

 calcareous earth, or separated in distinct concretions. When the 

 cavities of a sponge or of crustaceous animal admitted the siliceous 

 earth to enter, it appear? to have been infiltered from the chalk, in 

 the same manner as the nodules of chalcedony have been infiltered 

 into the cavities of lava or basalt. Between chalcedony and flint 

 there is a near resemblance ; they are only different modes of the 

 same substance, and the flint nodules in the western counties of Eng- 

 land are frequently chalcedonic. The hardest rocks and stones are 

 permeable to water ; flint, when first got out of the chalk is easily 

 fractured, and the fractured surface is found covered with moisture. 



The organic remains in the chalk formation are exclusively marine. 

 They are too numerous to be described in the present work, but it 

 will be proper to notice those that are the most characteristic. These 

 are, first, echinites, particularly the helmet-shaped species called 

 ananchytes, and the heart-shaped species spatangus, cor anguinum. 

 The chambered shells called scaphites, hamites, turrilites, and bacu- 

 lites, are regarded as peculiar to the chalk formation : it also con- 

 tains ammonites, belemnites, and nautilites. Numerous organic re- 

 mains of zoophytes, in the state of flint, particularly of sponges and 

 alcyonia, and various species of bivalve shells, occur in chalk ; but, 

 there are comparatively few spiral univalve shells in this formation. 

 It is probable that the deep ocean in which chalk was deposited, was 

 not suited to the inhabitants of such shells, for the animals had heads 

 and eyes, and required shallow water to see their food. Several 

 specimens of fossil fish from chalk may be seen in the valuable mu- 

 seum of Mr. Mantell, at Lewes, and some vertebral remains of large 

 saurian animals ; but these are rare. Teeth, palates, and scales of 

 fishes, occur, more frequently, in chalk, than vertebrae. The great 

 preservation in which some of the most delicate organic remains are 

 frequently found, render it probable that chalk was deposited in a 

 deep and tranquil sea. Balls of iron pyrites, with a radiated diverg- 

 ing structure, are frequently found in chalk ; and in the chalk-pits 

 near Dorking, the large spines of echini, of the genus Cidaris, are 



