SILICIOUS CONCRETIONS IN LIMESTONES. in 



In. the Firestone there is some volcanic ash. 



Chalk. Unbroken foraminifera make up but a small part of 

 the Chalk, and all the fragments of these organisms together would 

 not make up half the bulk of the rock. Prisms from the shells of 

 Inoceramus, portions of Ostrea, Pecten, polyzoa, and echinoderms 

 are abundant, as are spines and spiculse of sponges. Fragments of 

 brachiopods are rare. Coccoliths abound. In some localities there 

 is abundance of fine quartz sand. In the soft Upper Chalk the cells 

 of the Foraminifera are empty, in the hard chalk of Yorkshire they 

 are filled with calcite. 



The Tertiary limestones in this country are mostly fresh-water 

 deposits, and limited to the Isle of Wight ; unless we except the 

 marine bryozoa limestones of the Coralline Crag in Suffolk. 



Flints. 



Flints are concretions of silica which have been accumulated in 

 the strata, after their consolidation, by the solvent action of per- 

 colating waters, which have dissolved the substance of various 

 minute skeletons of silicious organisms, and re-deposited the material. 

 The chief accumulations of flint are met with in the Carboniferous 

 Limestone, in the Portland and Purbeck beds, and in the Chalk. It is 

 probable, in some cases, that no small amount of this silicious material 

 has actually been derived from the solution of overlying sandstones, 

 which have happened to contain sufficient lime to render the silica 

 soluble. In the Carboniferous Limestone, the whole substance of the 

 rock is sometimes removed and replaced by flint or chert, which 

 exhibits cavities formerly occupied by fossils. In the Portland 

 beds, the black flint is sometimes found in horizontal, sometimes 

 in inclined fissures ; and it frequently fills the interior of certain 

 fossils, such as the Cardium dissimile. In the Upper Greensand of 

 Warminster, and tire Greensand of Black Down, the substance of 

 many shells has been completely replaced by silica, and this con- 

 dition is often met with in the Thanet Sands and other deposits. 

 The most typical exhibition of flints, however, is furnished by the 

 Chalk. The amount of flint in the Chalk varies with the locality, 

 and is nowhere more remarkable than in the neighbourhood of 

 Axmouth and Beer Head, though the individual flints are relatively 

 small. Their number is, however, so great as sometimes almost to 

 obliterate the appearance of arrangement in horizontal layers. 

 Though chiefly characteristic of the Upper Chalk, flints are not 

 exclusively limited to it, but they never occur in bands of nodules 

 in the Lower Chalk. The size of the nodule, and the distance apart 

 of the horizontal layers, which mark stratification, are extremely 

 variable. The nodules themselves are of very irregular form, always 

 white on the external or growing surface, where it is in contact with 

 the chalk, and black or blackish in the interior. They are well 

 developed in the Chalk of Norfolk, and the softness of the chalk is 

 usually in proportion to the development of flint nodules. 



Near London the nature of flints may be easily studied in the chalk 



