Page:The Journal of geology (IA journalofgeology11893univers).pdf/85
"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 currents the fresh and salt water mingle, and the expanse of brackish 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 freshened at a similar distance from the La Plata. This thin sheet of brackish water is exposed to variations of temperature and barometric 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 equilibrium, 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, undissolved and unused by organisms, and will ultimately be deposited as calcareous ooze.
Oceanic circulation maintains an approximately uniform composition 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 suspension. Thus the lime brought down by rivers, though measurable by hundreds of thousands of tons per annum, is so widely diffused in the vast volume of the ocean that it escapes recognition.
There are, however, several instances of modern limestone formation which, though local, illustrate the processes of chemical deposition on a large scale. The descriptions of these may