SOURCE OF THE SILICA 599 



silica. The forty-five streams in the eastern part of the United States 

 that show more than 10 per cent silica have an average of 102 parts per 

 million of the total dissolved solids, of which 19.2 parts are silica. 



Sufficient evidejice has been cited to indicate that most streams drain- 

 ing areas of heterogeneous rocks are high in silica. The single analysis 

 available of the present streams in the area under discussion shows only 

 8 per cent of silica, but the data given above indicate that ]iot improbably 

 the amount of silica in the streams of tlie past was much higher. It is 

 not necessary that there should have been a larger amount of silica, how- 

 ever, for quantitative estimates of the amount of silica in the oolitic beds 

 indicate that the present percentage, 8 per cent, would be adequate for 

 the formation of the oolites. 



Cause of the Precipitation of the colloiDxIl Silica as Oolites 



It is well known that silicic acid, the form of the colloidal silica in 

 streams, is readily coagulated on mingling with an electrolyte. Such an 

 electrolyte would be furnished by the brackish or saline waters of the sea 

 into whicli the streams were emptying. An excessive amount of salts in 

 the water would cause rapid coagulation, which would result in the pro- 

 duction of oolites. A series of experiments to determine the extent to 

 which ordinary sea-water would coagulate the amount of silica in solution 

 in streams, made in connection with the writer's studies on the origin of 

 chert,^^ showed that silica is rapidly removed from the sea-water as it is 

 added by the streams. This coagulated silica is thrown down on the sea- 

 lx)ttom, and through surface tension assumes a spherical form. These 

 splierical forms are the result of tlie aggregation of the silicic gel mole- 

 cules Avhich are said ])y Tolman^^ to be 30,000 times as large as the ordi- 

 nary molecule. Possibly this coagulation and rounding would be aided 

 also by the agitation of the waters, as is suggested by Linck and others. 



These rounded masses would grow as they settled through the water 

 and after they came to rest on the bottom. The smaller oolitic grains 

 apparently represent the first product of the coagulation of the silica. 

 Tlieir remarkably uniform size, amount, and distribution all favor this 

 view. As a rule, these small oolites carried down more clay than was later 

 included in their outer layers. The later growth occurred wherever silica 

 was sufficiently abundant in the adjacent clay. If this rate of growth was 

 slow, then well defined rings resulted, but if the rate was fast no rings 

 I'csulted. Factors controlling this growth would be variations (a) in the 



." W. A. Tarr : Am. Jour. Sci., 4tli ser., vol. 44, 1917, pp. 409-452. 

 iM\ E. Tolman, .Ti-.. and J. D. Clark: Econ. Geol.', vol. 9, 1914, p. 561. 



