138 THE FORMATION OF FLINTS 



animal group known as the flagellate Infusoria.* Forami- 

 nifera, fragments of shells, and other calcareous debris 

 make up the remainder of the calcareous constituents 

 in the case both of chalk and grey ooze. With so many 

 characters in common, we might fairly expect to find 

 flints also in both; but in this we should be disappointed. 

 Flints have never yet been found in the grey ooze or in 

 any other deposits of the existing sea. We will turn 

 aside, therefore, for a short space to investigate the flints 

 themselves a little more closely. 



If we break open a flint we find that it is composed, 

 for the most part, of a hard, black substance so hard 

 that it cannot be scratched by a knife and if the sharp 

 edges of a broken flint and a blade of steel are struck 

 together, minute splinters of the steel fly off, and some of 

 these are heated by the blow to such a pitch that they 

 take fire and burn in the air. The high temperature is 

 due to the fact that in striking sharp edges together the 

 energy of the blow is concentrated over a very small 

 area, and so likewise is the resulting heat. This is the 

 philosophy of a " strike-a-light." The substance of the 

 flint is very homogeneous, with no pronounced tendency 

 to break in one direction rather than another, resembling 

 glass in this respect, so that it gives under a blow with a 

 conchoidal fracture i.e., with an undulating surface. 

 If struck full face by the corner of a hammer, a cone 

 of fracture extends into the mass, or a number of cones 

 surrounding the same axis. These are revealed as con- 

 centric circles on the surface of a pebble which has been 

 battered in a stream. It is the homogeneous nature of 

 the flint, combined with its great hardness, which has 

 rendered it so serviceable in the fabrication of imple- 

 ments. Flint, again, is highly insoluble. Rain-water 



* H. Lohmann, "Die Coccolithophoridae, Arch. f. Protistenkunde," 

 Bd. I., 1902, p. 89. 



