!9 18 J Davis: The Bacliolarmn Cherts of the Franciscan Group 301 



Hovey 33 has also described similar alterations of chert. He states 

 that in certain cherts of Missouri, the chemical difference between the 

 fresh and the altered chert is very slight, but the physical difference 

 is remarkable. He describes altered chert as extremely porous and 

 breaking easily between the fingers. All gradations in the process of 

 alteration are to be found. Some chert is perfectly fresh except for 

 an outer shell of decomposition, while other specimens are altered 

 throughout their whole mass. The altered chert is sometimes spoken 

 of as "tripoli. " 



Russell 34 describes similar occurrences in certain cherts of Ten- 

 nessee and Alabama. In the Silurian and Carboniferous limestones 

 the flinty concretions are exceedingly hard and apparently highly 

 durable when fresh. When exposed to the atmosphere, they crumble 

 to a fine white powder, bearing no resemblance to the original rock. 

 The change is abrupt, but the original homogeneity is shown by con- 

 centric lines and bands of various tints which run without interrup- 

 tion through both the solid and friable portions. 



The flints from the chalk of England 35 show similar changes. 

 Between crossed nicols, the flint shows a fine mottled appearance like 

 that exhibited by certain types of chalcedonic silica. While a micro- 

 scopic study of thin sections of flint shows little evidence of the pres- 

 ence of opal ; still on treatment with caustic potash, opal may be dis- 

 solved out so that the flints appear to consist of a mixture of chalce- 

 donic and opaline silica. When treated with a hot solution of caustic 

 potash the opaline silica is removed, leaving the chalcedonic silica 

 behind in a firm porous mass which retains the original form of the 

 flint fragment. 



If a flint from the chalk be treated with acid until all the adherent 

 lime carbonate is removed, it will be found to be coated with a pure 

 white rind of porous silica, apparently due to the weathering of the 

 flint. In such specimens it will be noted that the line of junction 

 between the porous white silica and the black flint is extremely sharp. 

 It is also a fact that if a freshly broken black flint be exposed to the 

 weather for a number of years, it gradually becomes covered with a 

 white coat of porous silica. 



33 Hovey, E. O., A Study of the Cherts of Missouri, Am. Jour. Sci., vol. 148, 

 p. 401, 1894. 



34 Eussell, I. G, U. S. Geol. Surv. Bull. 52, p. 21, 1889. 



35 Sollas, W. J., The Origin and Formation of Flints, in The Age of the Earth, 

 pp. 132-65, London, 1908. 



Hill, W., Flint and Chert, Proc. Geol. Assoc., vol. 22, p. 61, 1911. 



