164 THE FORMATION OF FLINTS 



into solution, for it very readily dissolves in alkalies, and 

 its removal would leave behind numerous minute cavities, 

 which would increase the area of the flint exposed to the 

 attack of the potash, and solution would consequently be 

 accelerated. 



If, however, opal were present to the extent of even 

 10 per cent, only, we should expect to find some other 

 signs of it. Its refractive index is so much lower than 

 that of quartz that it should be visible under the micro- 

 scope, but it has never yet been detected. Again, opal, 

 by virtue of its porosity, readily absorbs coloured fluids, 

 so that it may be readily made. visible in rocks containing 

 it by staining ; but prolonged immersion of flint in alco- 

 holic solutions of aniline dyes reveals only the merest 

 trace of opal, scarcely 1 per cent., if we may judge by 

 mere inspection. On the other hand, as we have seen, 

 opal stains owing to its porosity, and if its pores were 

 already filled with crystalline silica, staining fluids would 

 be excluded, so that the argument drawn from the absence 

 of staining is not absolutely conclusive. 



These difficulties give increased interest to the obser- 

 vations of Messrs. Michel Levy and Munier-Chalmers, * 

 who, after a searching investigation into the properties of 

 chalcedony (which agrees as regards specific gravity and 

 solubility with flint) and other forms of fibrous silica, 

 have been led to the conclusion that in these substances 

 we are presented with a third form of crystalline silica 

 (tridymite and quartz being the other two). It differs 

 from quartz in being optically bi-axial, and by a slightly 

 higher degree of bi-refringence, as well as in its specific 

 gravity, which is that of flint. These observers, however, 

 and subsequently Mr. Wallerand, I suggest that quartz 



* I have to thank my friend and colleague, Professor Miers, for 

 calling my attention to these observations. 



f Bull Min. Fr. Soc., vol. xv. p. 159, 1892, vol. xx. p. 52, 1897 



