THE MINERAL MATTER OF THE SEA 471 



the zone of katamorphism — and limestone is known to exist there — 

 all the above figures will be increased. 



No careful estimate of the amount of limestone beneath the sea 

 is possible, but its amount must be great. It probably forms a 

 larger proportion of the sediment than on land, but it does not follow 

 that its average thickness is greater. If we assume that the average 

 thickness of limestone beneath the sea is half as great as that on the 

 land, and that the amount of calcium carbonate in other sedimentary 

 rocks beneath the sea is, on the average, half as great per square mile 

 as on land, we may derive from the figures representing the estimated 

 thickness of limestone material on the continents, figures representing 

 an estimated average for the earth. These figures are 420 (based 

 on the estimate of Van Hise) and 850 feet (based on the estimate of 

 Dana). The estimate of Reade, increased to allow for the calcium 

 carbonate in clastic rocks, becomes 738 feet. 



Even if these large figures be correct, they represent less than the 

 total amount of mineral matter which has been extracted from the 

 sea, since they deal with one sort of mineral matter only. How much 

 they should be increased to include all the mineral matter ever 

 extracted from the waters of the sea cannot be stated ; but the silica, 

 the various sulphides and sulphates, the chlorides, etc., which have 

 been extracted from the sea water, would swell them appreciably. 



Much material, such as that of limestone, has been extracted 

 from the sea water and deposited, and then re-dissolved, re-extracted, 

 and re-deposited. Some material, indeed, has probably gone through 

 this cycle many times. The aggregate result of the solvent work of 

 water is therefore not represented by the amount of existing rock 

 matter which has been extracted from the sea, plus that which still 

 remains in solution. Furthermore, the considerations adduced take 

 no account of the deposition of material from solution on the surface 

 of the land, or beneath it, or in the lithosphere under the sea. The 

 amount of mineral matter deposited from solution in these situations 

 is certainly great, though it cannot well be estimated. It must, how- 

 ever, be recognized, in attempting to gain the proper conception of 

 the solvent work of ground water. 



2. By comparing the mineral matter in the sea water with that 

 in average river water, Tables I and II, it is seen that calcium car- 



