MASS OF THE SEDIMENTS. 939 



Various estimates, which are little more than guesses, have been made 

 of the thickness of the sedimentary rocks upon the earth. Dana in 1875 

 stated that the mean thickness of the sediments for the continental areas 

 will not exceed 5 miles (8.05 kilometers)." T. Mellard Reade states that an 

 estimate of 10 miles (16.1 kilometers) is a moderate one for the thickness 

 of the sedimentary crust of the earth throughout.'' Later he greatly reduces 

 this estimate, saying: "I think we may with safety provisionally assume 

 that the actual average thickness of the sedimentary crust of the globe is 

 not less than 1 mile (1.61 kilometers)." This average is for the globe as 

 a whole, and not for the continental areas, and therefore is not greatly 

 below Dana's estimate of 8 kilometers for the continents alone. It appears 

 to me that even Reade's revised estimate is much too great. At only a 

 very few localities can it be shown that the thickness of the sediments 

 approximates 8 kilometers. For the great plains and plateau areas where 

 there are deep borings or deep gorges, as, for instance, in the Grand Canyon, 

 the sediments frequently do not exceed 1 kilometer, and in g-eneral are less 

 than 2 kilometers. For great areas of the continents, such as extensive 

 Archean areas of Canada, there are no known sediments. For the greater 

 part of the ocean, aside from the continental shelves, all the evidence avail- 

 able points toward very slow deposition, and over such areas, composing 

 about two-thirds of the surface of the globe, the sediments are probably of- 

 inconsiderable thickness. 



The area of the globe is approximately 500,000,000 square kilometers, 

 of which somewhat more than one-fourth belongs to the continents and 

 continental shelves. If it were supjjosed that the sediments which have 

 not been within the zone of anamorphism, but have always remained in 

 the zone of katamorphism, are upon an average 2 kilometers thick for the 

 continents, this would give 250,000,000 cubic kilometers of sediments. 

 This estimate is, of course, little more than a guess, made in order that a 

 number of problems in the distribution of the elements may be quantita- 

 tivel} r stated. In comparing- this estimate with that of Dana or Reade it 

 should also be noted that mine includes only sediments of the zone of 

 katamorphism — not those which have been metamorphosed in the zone 

 of anamorphism — while Reade's estimate includes both. 



"Dana, James D., Manual of Geology, second edition, 1S75, p. 657. 



6 Reade, T. Mellard, Chemical denudation in relation to geological time, p. 29. 



«Loc. cit., p. 52. 



