896 J. BARBELL MEASUREMENTS OF GEOLOGIC TIME 



led to wide-spread and possibly long-enduring epeiric seas which have left 

 no record, because a somewhat later sinking of the sealevel would have 

 resulted in a washing of the previous deposits from the land. 



The conclusion is so important that for better visualization it may be 

 subjected to a quantitative statement in so far as the inexact nature of 

 the data lends itself to such statement. The total volume of average 

 igneous rock needed to supply the oceanic sodium is 84^300,000 cubic 

 miles. This allows for the incomplete leaching of the igneous rock dur- 

 ing erosion and sedimentation.^*^ Take, as the extreme assumption, four- 

 fifths of this as eroded in Precambrian time. This gives 67,000,000 cubic 

 miles, the maximum which can be granted. Take the length of the Pre- 

 cambrian as 670,000,000 years, a minimum figure as given by uranium 

 minerals. The result is an average of 0.1 cubic mile of igneous rock 

 eroded per year — a maximum estimate. 



To compare this average for the Precambrian with the present rate of 

 supply of new salt to the sea, take the present area of igneous rocks as 20 

 per cent of the land area, or 11,000,000 square miles, as measured by 

 Von Tillo. Take the present mean rate of erosion as one foot in 8,600 

 years, as given by the measurements of chemical denudation, and consider 

 it to apply ^ the regions of igneous rock. This gives an erosion of .24 

 cubic mile per year of igneous rock. 



A check calculation is given by taking the sodium uncombined with 

 clilorine which is carried by the rivers to the sea each year. This is 

 69,000,000 tons. Let this be taken as a measure of the new sodium de- 

 rived annually from the erosion of igneous rocks. In so far as new 

 chlorine from volcanic action is combined with new sodium in the soil, 

 this figure may be below the truth and tends to be a minimum, l^ow one 

 cubic mile of average igneous rock will, by being subjected to erosion, 

 yield 200,000,000 to 210,000,000 tons of sodium. The 69,000,000 tons 

 of unchloridized sodium represents, then, the erosion of 0.33 cubic mile 

 of average igneous rock. Thus we may take the mean of the two estimates 

 and conclude that not less than 0.3 cubic mile is eroded per year now as 

 compared to not more than 0.1 cubic mile as the average for Precambrian 

 time. 



But the present supply of new sodium is from only 20 per cent of the 

 land area. In the Precambrian the average area of igneous rocks exposed 

 was much greater, though at times considerable portions of the continents 

 were covered in part by shallow sea and mantles of sediment. If the mean 

 area of exposure of igneous rocks was then four times the present area. 



148 F w. Clarke : Data of geochemistry, p. 30, Bull. 616, U. S. Geol. Survey, 1916. 



