E. 0. Hovey — Cherts of Missouri. 407 



The amounts given above for the Missouri cherts are below 

 these maxima and, therefore, do not necessarily indicate the 

 presence of opal (amorphous) silica, since chalcedony is held 

 to have the same chemical characteristics as quartz. When a 

 large percentage of the silica is soluble in caustic potash the 

 presence of opal silica is indicated ; e. g., the Arkansas Survey 

 chemist found 30 - 72 per cent of a Silurian chert, 35*56 per 

 cent of a Tertiary chert and 88*38 per cent of a geodized coral 

 from Tampa, Florida, to be thus soluble. These geodized 

 corals are known to be opal and the two cherts must contain 

 large amounts of amorphous silica. Edward T. Hardman* 

 analyzed a series of twelve specimens of chert from the 

 Upper Carboniferous strata of Ireland. He tested the rocks 

 in hydrochloric acid and found traces of soluble silica in sev- 

 eral and 1 per cent, 1 22 per cent and 1*5 per cent in three 

 cases. All the Cretaceous chalk flints contain much opal 

 silica and show high percentages of silica soluble in caustic 

 potash.f 



It will be seen from the table that the chemical difference 

 between " altered " and " unaltered " chert is so very slight 

 that they can be distinguished only by physical characteristics. 

 The most completely altered chert is that from Seneca, which 

 is minutely porous, breaks to pieces readily between the 

 fingers and may be ground to an impalpable powder in an 

 ordinary mill. There is an extensive bed 18 ft. thick of this 

 material at this locality and it is quarried for the manufacture 

 of filtering disks and tubes and of a high grade of polishing 

 powder. A similar rock occurs near Seneca in the Indian 

 Territory, at Dayton, Newton Co., Mo., and in township 4 S., 

 26 W. in central Arkansas.^; The Arkansas rock disintegrates 

 to a fine powder on exposure to the atmosphere, but the Seneca 

 rock does not. Another item of interest in the analyses is the 

 very low percentage of water (by ignition) which was found, 

 especially in the pure cherts, whether altered or unaltered. 

 This would argue against the presence of more than a very 

 small amount of opaline silica. Mr. Hardman's analyses of 

 the Irish Carboniferous cherts brought out the same fact 

 regarding the presence of water. 



Origin of the chert. — There has been much speculation as 

 to the origin of flint, hornstone and chert. The Cretaceous 

 flints of the chalk formation in England and elsewhere con- 

 tain so many remains of originally siliceous animal organisms 

 (skeletons of siliceous sponges and polycystines) that some 

 authors claim that all their substance has come from this source, 



* Sci. Trans. Roy. Dub. Soc, I, vol. i, pp. 85, 1878. 

 •f- Vid. Geology, J. Prestwich, vol. ii, pp. 321 and 322. 

 % Ark. Geol. Surv., Ann. Rep., 1890, vol. iii, p. 384. 



