MASS OF CALCAREOUS SEDIMENTS. 941 



which I am aware of on the subject is in reference to limestone. T. Mellard 

 Reade, by comparing' the amount of calcium carbonate and calcium sulphate 

 in solution in rivers with the sediments mechanically carried by rivers, 

 concluded that one-eighth to one-ninth of the transported material is com- 

 posed of the carbonate and sulphate of calcium. Using- his estimate of a 

 thickness of 1 mile for the sedimentary rocks, and supposing that the mass 

 of limestone is one-tenth of the whole mass of the rock, he says: "This 

 gives us the equivalent of a zone of limestone rock 528 feet (160.9 meters) 

 thick, enveloping the globe, as a very rude approximation to the absolute 

 quantity of carbonate and sulphate of lime in the sedimentary crust of the 

 earth." ° Of the two compounds calcium carbonate so dominates over 

 calcium sulphate that the latter will be neglected and the whole will be 

 considered as calcium carbonate. 



If the amount of calcium carbonate carried by the streams at present 

 is not an accurate measure of the averag'e carried by streams thr.ough geo- 

 logical time, the error is likely to be an overestimate rather than an under- 

 estimate, since early in geological times, before the calcareous sediments 

 had accumulated, it can not be supposed that the streams carried as much 

 calcium carbonate as at present. Further, the method of computation used 

 by Reade for estimating the volume of the limestones is rather unsatisfac- 

 tory, since it is certain that a considerable portion of the calcium carbonate 

 passes into shales and sandstones, as is shown by the analyses (p. 938). 



The shales contain 2.64 per cent of carbon dioxide and the sandstones 

 3.03 per cent. If most of this carbon dioxide is united with calcium oxide, 

 as is probable, this would give about 5 per cent, or one-twentieth, of calcium 

 carbonate in the shales and sandstones. Therefore, if the shales and sand- 

 stones were supposed to be nineteen times as abundant as the limestones, 

 these rocks would require one-half of the calcium carbonate, and the propor- 

 tion of limestone instead of being one-tenth, as estimated by Reade, would 

 be one-twentieth of the total mass of the sediments. This ratio of 19:1 

 between the sandstones and shales together and the limestones is accordant 

 with the relative amounts of the rocks as calculated upon the basis of 

 depletion of calcium oxide in the sandstones and shales as compared with 

 the original rocks (see pp. 990-991). It thus appears that the estimate of 

 one-twentieth, or 0.05, of the mass of sediments for limestone, is probably 

 as near the truth as can be made from data at the present time. 



"Reade, T. Mellard, Chemical denudation in relation to geological time, p. 53. 



