It may be assumed that the average water of the ocean has not been of the same composition in any two of the geological periods. The ocean has had a chemical evolution. Though the amount of water may have remained tolerably constant from the pre-Cambrian time to the present, it seems certain that the amount and character of the mineral matter dissolved in the ocean must always have been changing. The inflow of river-borne salts has varied in rate according to the area of the lands and according to the kinds of rocks exposed to subaerial erosion during the different periods. On the other hand, there are reasons for believing that the precipitation of salts from the sea-water (including biochemical precipitation) has occurred at variable rates. The composition of the existing average sea-water may conceivably be dependent on four other factors: a, the primitive condition of the ocean before . . .