CHERT FORMATIONS OF NOTRE DAME BAY 597 



silicate dissociate and yield colloidal silica. Clarke^ in reviewing 

 their work concludes, "in natural waters, then, silica is actually- 

 present in the colloidal state, and not in acid ions." 



That silica is flocculated or precipitated by ocean water is clearly 

 shown by its absence in the ocean water and its abundance in river 

 water. Precipitation in this case is clearly brought about by the 

 action of electrolites. If magmatic waters were discharged into 

 ocean water this action would surely prove effective. 



Tarr has suggested^ a similar method of coagulation for the 

 origin of some syngenetic chef t nodules, assuming that the silica was 

 carried into the sea by rivers. Dean^ in criticizing this theory says: 



If, however, the silica is precipitated by the salts of the sea, these salts, 

 or at least their cations, should be the principal material to be absorbed by 

 silica and would hence form the principal impurity in the chert. Since sodium 

 chloride is the salt present in largest amount in the sea water, we should expect 

 to find a predominance of sodium in the chert; as a matter of fact, the analyses 

 show very little sodiuxn and a decided predominance of calcium and magnesium 

 carbonates.'* 



In answer to this criticism it seems doubtful to the present 

 writer if the concentration of alkahes in sea water as compared to 

 the concentration of the alkaline earths is sufficiently high (7.3:1) 

 to greatly overbalance the far greater coagulative power of the 

 bivalent ions. The analysis on page 580 shows 1.04 per cent of 

 alkalies and .73 per cent of alkali earths in molecular ratio 

 Na+K:Mg+Ca: :i.6:i. The absolute figures and the ratio 

 appear to the writer not inconsistent with theory of coagulation 

 by electrolites. The abundance of K is perhaps to be explained 

 by the displacement of adsorbed Na. 



It seems reasonable to suppose that the magmatic waters derived 

 from the same sources as the volcanics of Notre Dame Bay would 



^ F. W. Clarke, "The Data of Geochemistry," U.S. Geol. Survey Bull. 616, 1916, 

 p. 194. 



^W. A. Tarr, "Origin of Chert in the Burlington Limestone," Am. Jour. Sci., 

 4th ser., Vol. XLIV (1917), pp. 409-52; "Oolites in Shale and Their Origin," Bull. 

 Geol. Soc. Am., Vol. XXIX (1918), pp. 587-600. 



3 S. Dean, "The Formation of the Missouri Cherts," Am. Jour. Sci., 4th ser. 

 Vol. XLV (1918), pp. 411-15. 



^ Ibid., p. 412. 



