140 THE FORMATION OF FLINTS 



crystal, or Bristol diamonds. Like, these, flint is truly 

 crystalline, but the crystals of which it consists are very 

 minute, and crowded together in confusion, so that they 

 do not present crystal outlines. When a flint nodule is 

 hollow inside, as often happens, the silica sometimes 

 takes advantage of elbow-room, and, being free to develop, 

 assumes the characteristic form of rock-crystal. 



If we extract a flint nodule from its place in the chalk, 

 and clean its surface of all adherent material, as by 

 washing, we shall find that its exterior is white and 

 porous, like the chalk itself. We may remove the last 

 traces of chalk by placing the flint in acid hydro- 

 chloric acid for choice but on washing again with w 7 ater 

 and drying, the surface is still white as before. On 

 breaking the nodule open we shall find, as a rule, that 

 the whiteness of the exterior is confined to a thin super- 

 ficial film, coating the internal mass of black flint, though 

 this itself is not uniformly black all through, but spotted 

 here and there with greyish blotches, some whiter, some 

 darker, with every degree of transition between the 

 lightest-coloured and the black flint. So much is 

 generally true, but the differences in detail which dis- 

 tinguish nodules from different localities, or even from 

 one and the same bed, are many and great (Fig. 38). It 

 is not at all uncommon to find nodules which are hollow 

 within, the enclosed cavity being filled with some other 

 material. Frequently this is a white, chalky powder, 

 which we shall examine later" in detail ; sometimes it is 

 the skeleton of a sponge, and in this case an interval 

 may occur between the exterior of the skeleton and 

 the surrounding envelope of flint. The interval may be 

 filled with a chalky powder of the same nature as that 

 just mentioned, or it may be empty and lined with pro- 

 jecting crystals of quartz. Again, in some cases there is 

 no interval, but a mass of greyish-white, or even black, 



