434 Tiirr — Origin of the Chert in^ the Burlington Limestone. 



Sea water is an electrolyte, hence there is a tendency for the 

 silica to be deposited near the shore, if sufliciently concen- 

 trated, and sncli disposition undoubtedly occurs. 



The direction and distance of transportation will depend 

 upon the currents. In shallow epi-continental seas, currents 

 are probably stronger than in deeper bodies of water. The 

 Burlington limestone shows cross-bedding and this with the 

 lenticular character of the beds suggests a shallow sea. Evi- 

 dently the currents were strong enough to produce cross-bed- 

 ding. Broad, shallow interior seas of warm water would 

 favor the development of currents. 



Presumably the salinity of these seas was not greatly differ- 

 ent from that of the present oceans, and, therefore, the wide 

 distribution of the chert was made possible because a very high 

 salinity would have meant rapid deposition of the silica near 

 shore. 



Another important consideration that would influence the 

 distribution of the silica would be the character of the water 

 near shore. It is well known that the saline water is suffici- 

 ently dilute to act as an electrolyte and has a marked influence 

 upon the rapidity of sedimentation, the fine clays and silts 

 being rapidly thrown down in saline waters while they remain 

 suspended for a far longer period of time in fresh water. If 

 the waters were receiving considerable clayey material it would 

 be deposited relatively near shore. The colloidal silica present 

 in the waters bringing in the clay and silts would be coagu- 

 lated by the electrolyte and carried down by the sinking clays 

 and silts, because even a very small amount of the gel would 

 adhere to the fine silts. When the waters were relatively free 

 from such fine silts the currents would transport the colloidal 

 silica to a much greater distance. 



S. Deposition of the Silica. 



(a) Experimental. — In order to determine to what extent 

 a water of the composition of the sea water would cause coagu- 

 lation of the silica brought to the sea by streams, a series of 

 experiments were made with solutions which would approxi- 

 mate those of the sea water and streams. As bivalent ions are 

 relatively more effective in causing coagulation than univalent 

 ions, solutions without bivalent ions were tried as well as those 

 containing them. While these experiments were not exhaus- 

 tive, they were in sufficient detail to show conclusively the 

 character of the reaction in such solutions. The solutions 

 representing the sea water contained the amount of salts in 

 solution by weight as found in sea water, that is, 34,400 parts 

 in 1,000,000 parts of water. Not all the ions present in the 



