252 WARREN J. MEAD 
obtained, certain suppositions have been made regarding the com- 
position and abundance, based on the best evidence at hand. Van 
Hise* estimates the average thickness of the sediments over the con- 
tinental areas as 2 kilometers, or about 6,000 feet, and their total 
mass aS 675,000,000,000,000,000 metric tons. This estimate is well 
in accord with Salisbury’s? estimate of one mile for the thickness of 
the sediments. The salts of the sea, computed as oxides from Ditt- 
mar’s determination, amount to a total mass equal to about 3.5 per 
cent. of the total mass of the sediments, as estimated by Van Hise. 
It is assumed that the average thickness of the residual mantle over 
the continental areas is 60 feet, or 1 per cent. of the total thickness 
of the sediments, and as a rough approximation the thickness of the 
iron formations, if spread uniformly over the continents, is 60 feet, 
or I percent. For the chemical composition of the residual material 
an average was taken of several analyses of residuary materials given 
by Merrill.3 For the average composition of the iron formations an 
average was taken of several hundred analyses of Lake Superior 
iron formations, weighting the ratio of iron-carbonate to iron-silicate 
rocks in the proportion of seven of the former to three of the latter. 
It is not assumed that the figures for composition and abundance 
here employed represent very closely the facts, but they are sufficient 
to show, in a rather broad way, the effect which a consideration of 
these minor end-products would have on the discrepancies arising. 
TABLE V 
: Av. Comp. of Ay. Comp. of Iron 
_Comp. of : Mineral Matter Ob ResidualsMateriall Formations. 
Sediments 82:12:6 |the Sea. Calculated Assumed 1% of Aesumcdr tor 
100% 3-57% of Sediments Sediments ; Sediments 
DIORA read aan yates 63.64 00.00 61.4 I.40 
3 5 5 
AIG OR erent oaier 14:71 00.00 21.04 Seri 
Nee ee Boy ors nna 4.46 00.00 8.25 28.50 
Mio. Onin ee See: 2.90 TORT ©.99 5-38 
CaO ye teieavaarcr cette ogy! 2332 0.98 3.82 
NTA @) eet oes sees I.20 81.76 0.81 0.00 
ICAO We a eunmenra ere 32 2.64 1.84 0.00 
1 C. R. Van Hise, A Treatise on Metamorphism, Monograph No. 47, U. S. Geo- 
logical Survey, 1904, p. 940. 
2 R. D. Salisbury, ‘‘ Mineral Matter of the Sea,’’ Journal of Geology, Vol. XIII, 
Septembler-October, 1905. 
3G. P. Merrill, Rocks, Rock-Weathering, and Soils, p. 306. 
