CONDITIONS OF SEDIMENTARY DEPOSITION. 5 1 1 



"One sees that the solubility in ocean water, itself very feeble, 

 is also notably more feeble than the solubility in fresh water." 



When river water enters salt water it is exposed in different 

 form and under different physical conditions from those which 

 existed in the river. As the fresh water is lighter than the salt, 

 it floats upon it and spreads out in a sheet not unlike a fan. As 

 compared with its depth and width in the river the layer is very 

 shallow and widens from the mouth. Through waves and cur- 

 rents the fresh and salt water mingle, and the expanse of brack- 

 ish water may be of great extent. Forchhammer attributes the 

 minimum salinity which he found for surface water from the 

 north Atlantic, 900 miles from the mouth of the St. Lawrence, 

 to the volume of that river, and he found the ocean water fresh- 

 ened at a similar distance from the La Plata. This thin sheet of 

 brackish water is exposed to variations of temperature and baro- 

 metric pressure produced by changing winds, now on, now off 

 shore, and is in constant agitation with the rise and fall of waves. 

 Thus the conditions which produce varying tension of carbonic 

 acid, and which aid the escape thereof, exist at its surface, and 

 the bicarbonate of lime in solution must be in unstable equili- 

 brium, with constant formation of neutral carbonate and more or 

 less constant recombination of it. If the neutral carbonate be 

 present in sufficient quantity it will remain in suspension, undis- 

 solved and unused by organisms, and will ultimately be deposited 

 as calcareous ooze. 



Oceanic circulation maintains an approximately uniform com- 

 position of ocean water in all parts of the open seas, and great 

 currents sweeping past river mouths distribute the contribution 

 of fresh water and its solid matters, whether in solution or sus- 

 pension. Thus the lime brought down by rivers, though meas- 

 urable by hundreds of thousands of tons per annum, is so widely 

 diffused in the vast volume of the ocean that it escapes recog- 

 nition. 



There are, however, several instances of modern limestone 

 formation which, though local, illustrate the processes of chemi- 

 cal deposition on a large scale. The descriptions of these may 



