Tarr — Origin of the Chert in the Burlington Limestone. 43T 



As consolidation proceeded the colloidal silica crystallized in 

 part as chalcedony, and in smaller part as quartz, leaving a 

 very small amount of residual colloidal silica as opal. 



When the waters contained much silt and clay, whose ions, 

 like those of the silica, carried negative charges and were thus 

 precipitated through neutralization, the silt and clay w^ere car- 

 ried down along with the coagulating colloid. The colloid had 

 a marked tendency to aggregate around the small, solid parti- 

 cles and bind them together, thus causing the rapid sedimenta- 

 tion of the silts. An abundance of silt, therefore, meant that 

 the major part of the colloidal silica would be removed from 

 solution before it could be carried to the more distant parts of 

 the interior sea which were relatively free from clastic material. 

 The great abundance of plastic clays (plasticity in clays is 

 largely due to colloids) and siliceous shales probably owe their 

 origin to this tendency of colloids to be carried down with 

 silts. 



Yery low^ lands, where chemical denudation exceeds mechan- 

 ical, would mean relatively clear waters at the margins of the 

 seas and this would permit the dispersal by shore currents of 

 the abundant colloids added to the seas by the streanas. Such 

 silica would be deposited with the limestone. An abundance 

 of silts carried into the sea water would mean the deposition of 

 most of the colloidal silica near the shores and the deposition of 

 relatively pure limestone beyond. 



(c) Form of the precipitated silica — The result of the 

 coagulation was the production of masses which were probably 

 spherical. The microscope shows that the spherical form of 

 these early molecular aggregates was retained in the larger 

 masses. These parts are now largely chalcedony and are pres- 

 ent in great numbers in the chert. There is absolutely no 

 evidence that they are of organic origin. The spherical char- 

 acter of the "first aggregates was thus passed on to the larger 

 masses, primarily because of the effect of surface tension, 

 which causes the gel to assume such a shape. The spherical 

 aggregates increased in size as more material was added to 

 them while they were being carried along by the currents. 

 When the size of the gel-mass was sufficiently large to cause it 

 to sink it found its way to the bottom. 



If the waters were shallow, currents and waves might have 

 shifted and rolled the masses of gel about, thus tending to 

 form larger aggregates. This rolling aided in producing the 

 banding of the chert, but only the relatively small masses were 

 rolled about. Circular bands are found only in the smaller 

 aggregates, those not exceeding 8 to 10 inches in diameter. If 

 the currents were unable to move the masses, the latter would 

 have grown through mass action, the larger aggregates attracting 



