214 A FLAKE OF FLINT AND ITS HISTORY. 



apply to the shells of molluscs, and likewise to corale. It is 

 important to mention that tjie sponges found in flint were 

 not originally of the horny nature of our bath-sponges, but 

 were tb em selves composed of minute spicules and fibres 

 of silica, like the so-called Venus's flower-basket of our 

 modern seas. 



With regard to the mode of occurrence of flint, we have 

 first to mention that it is by no means confined to the 

 chalk, but may occur in limestones of any age. In this 

 country it is, however, more abundant and purer in the 

 chalk than in any other formation, and may, indeed, be 

 considered characteristic of the upper part of that forma- 

 tion.* Flints are found in the chalk either in the form of 

 nodules or in thin continuous laminae. The nodules are 

 generally of very irregular shape, and may vary in size 

 from a walnut to masses of a hundred- weight or so. As 

 a rule they occur in strings at comparatively regular 

 intervals in the chalk, generally conforming more or less 

 closely to the original planes of bedding, and the indi- 

 vidual nodules being sometimes at considerable distances 

 apart, but at others closer together and more or less 

 connected by long root-like pieces. On the other hand, 

 the laminated or tabular flint may cut the bedding-planes 

 of the chalk at any angle, and is often found in joints and 

 fissures, which may emerge at the surface. As a rule, this 

 tabular flint is devoid of organisms. Not unfrequently 

 flints may be found which near their surfaces gradually 

 become paler and paler in colour, and contain an increasing 

 amount of calcareous matter, till they pass imperceptibly 

 into hard siliceous chalk. Again, flints may be hollow, 

 and contain in their cavities either corals or sponges, or 

 masses of that variety of silica known as chalcedony. 

 The latter, it may be mentioned, is a semi-transparent 

 waxy-looking stone, generally with a more or less decided 

 pinkish tinge, and forming niammillated or botryoidal 

 masses. The milky and reddish varieties of chalcedony con- 



* It also occurs abundantly in the lower part of the Portland stone 

 series of the Isle of Portland, but is there generally less pure, and has 

 the conchoidal fracture less marked. 



