Vol. 50.] CERTAIN NOVACULITES AND QT7ARTZITES. 379 



The employment of a Klein's plate does not, in the case either of 

 Arkansas or Ouachita stone, appear to indicate the presence of any- 

 appreciable amount of amorphous silica. Even in flint the close 

 examination of single minute areas during a complete revolution 

 discloses a change of tint in the Klein's plate, and it therefore 

 appears that, at all events for the most part, the component particles 

 of flint are doubly refracting. 



So far as the presence of soluble silica in cherts is concerned, the 

 researches of Prof. Renard ' indicate that in those of the Carboni- 

 ferous Limestone of Belgium the amount is extremely small. More- 

 over, the analysis, by Mr. E. T. Hardman, of one of the purest 

 specimens of Irish chert examined by him gave 95"50 per cent, of 

 insoluble silica, while only a trace of soluble silica was present. 2 



On treating a thin translucent splinter of Arkansas stone with 

 fuchsine no appreciable staining was visible under the microscope. 

 "Without here questioning the opinions expressed by Mr. Griswold, 

 which appear to be borne out by the analyses given by him, it 

 appears to me strange that, although he notes the close resemblance 

 of some of the Arkansas stone to chalcedony, he says nothing about 

 the extremely close resemblance in microscopic structure between 

 it and flint. 



Prof. J. D. Dana defines flint as " somewhat allied to chalcedony, 

 but more opaque," and with regard to the silica of which it is corn- 

 composed, he adds that it, " according to Euchs, is partly soluble 

 silica." 3 Prof. Tschermak observes that it contains some opal sub- 

 stance, due to organisms. 4 Prof. A. de Lapparent, after alluding to 

 the fibrous structure of chalcedony, as seen under the microscope, 

 adds : " when chalcedony becomes very compact, with a more and 

 more confused orientation, it passes into flint." 3 Finally, we have 

 the researches of MM. Michel-Levy and Munier Chalmas, 6 in which 

 more exact methods of determining chalcedony and other forms of 

 silica are employed, but in cases such as those which we are now 

 considering no satisfactory results could be arrived at, so far as the 

 determination of optical characters is concerned — one difficulty being 

 that we are not dealing with fibres, but with granules of microscopic 

 dimensions and of most irregular forms, so that there is no crystallo- 

 graphic direction of elongation upon which to base observations. 



I have attempted to determine the optical sign in the apparent 

 direction of elongation of some of the larger siliceous grains forming 

 the border of a rhombohedral cavity in Ouachita stone, with the 

 result that only alternate grains give a rise, while the intermediate 

 produce a fall in the colour-scale when a quartz-wedge is employed. 



1 ' Recherches lithologiques sur les Phthanites du Calcaire Carbonifere de 

 Belgique,' Bull. Acad. roy. Belgique, ser. 2, vol. xlvi. (1878) p. 494. 



2 'The Chemical Composition of Chert, and the Chemistry of the Process by 

 which it is Formed,' Sci. Trans. Roy. Dublin Soc. vol. i. (new series) p. 90. 



3 ' Svstem of Mineralogy,' 6th ed. 1892, p. 189. 



4 ' Lehrbuch d. Mineralogie,' 3rd ed. 1888, p. 388. 



5 ' Cours de Mineralogie,' 1884, p. 340. 



6 ' Memoire sur diverses Formes afi'ectees par le Reseau Elementaire du 

 Quartz,' Bull. Soc. Min. France, vol. xv. (1892) p. 159. 



