436 Tarr — Origin of the Chert in the Burlington Limestone. 



Experiment 5. — In the fifth experiment half of the solntion 

 obtained in experiment 4 was taken and to it was added the 

 usual amount of MgSO^ found in sea water. The result was 

 an immediate and large increase in the coagulated material, so 

 that it resembled in amount and appearance the precipitate 

 formed when MgSO^ had been among the original constituents 

 of the water used, thus showing that a bivalent element has a 

 marked effect in causing an increase in the amount of gel 

 thrown down. 



These experiments are sufficient to show that quantitatively 

 and qualitatively the sea water is able to throw down the silica 

 added to it by the rivers. The silica is probably not precipi- 

 tated immediately but accumulates in the form of the colloid 

 until it is of sufficiently high concentration to be coagulated. 

 This explains the occurrence of the silica in layers at varying 

 intervals in the limestone. 



ih) Cause of precipitation. — The precipitation of the col- 

 loidal silica is believed to have been due to the coagulating 

 eifect of the saline water of the sea, which can be regarded as 

 essentially an electrolyte. The colloidal silica carrying a neg- 

 ative charge prevented the aggregation of the suspended col- 

 loidal particles, as they mutually repelled each other. The 

 alkalies and alkaline earths in the sea water carry positive 

 charges, so when the negatively charged ions of the colloidal 

 silica came in contact with these positively charged ions, the 

 negative charge was removed and the colloidal particles of 

 silica were able to unite into larger aggregates or to coagulate, 

 as it is generally called. With the removal of this electrical 

 charge the strong surface tension of the silica became an 

 important aid in bringing about a union of the dispersed 

 particles. 



The ions of solutions of sodium^ potassium, calcium, and 

 magnesium salts, all of which carry positive electrical charges, 

 are especially effective in bringing about the removal of the 

 negative charge from the colloidal silica ion. This is espe- 

 cially true of ions with two charges, like calcium and mag- 

 nesium. Recent work by Cox, Dean, and Gottschalk* has 

 shown that calcium bicarbonate is an important precipitant, a 

 fact which probably has an important bearing on the common 

 occurrence of chert and flint with limestones and dolomites. 



It is supposed that the coagulated particles assumed a spher- 

 ical shape as they united to form moleculesf and that these 

 molecules were further aggregated to form spherical masses. 



*Cox, G. H., Dean, G. S., and Gottschalk, V. H., Mining Experiment 

 Station, Mo. School of Mines, Bull. 2. vol. iii, pp. 9-15. 



fit is estimated:}: that the molecnles of colloidal silica ha\e a molecular 

 weight of 80,000, which indicates something as to their large size. 



tTolman, C. F., Jr., and Clark, J. D., Econ. Geol., vol. ix, p. 561, 1914. 



