FORMATION OF SEDIMENTARY ROCKS 241 
approximation to the average composition of the original igneous and 
crystalline rocks, making due allowances for the end-products other 
than sediments, not included in the combination. 
DATA USED 
There are only eight of the elements—oxygen, silicon, aluminum, 
iron, calcium, magnesium, sodium, and potasstum—which compose 
more than 1 per cent. of the outer part of the lithosphere. The 
least plentiful of these, potassium, makes up about 2.3 per cent. of 
the lithosphere, while the next element in abundance, titanium, makes 
up only o.4 per cent. The present discussion will be confined to the 
eight most abundant elements, which will be treated in the form of 
the following oxides: silica, alumina, magnesia, soda, and potassa. 
Tron will be considered in the metallic form, the sum of the metallic 
content of the ferrous and ferric oxides being taken in each analysis 
employed. 
Perhaps the best determination of the average crystalline and 
igneous rocks yet available is one made by Clarke.t In obtaining this 
average the rock-forming elements were considered separately, each 
being averaged according to the actual number of determinations made. 
A group of truer estimates was thus secured than by averaging com- 
plete analyses together, as he had heretofore done. In the same bul- 
letin (p. 21) are average analyses of the several sedimentary rocks. 
The average analysis of the shales represents an average of seventy- 
eight rocks carefully selected and weighted. Two determinations of 
the average composition of the sandstones are given. The average 
represents a composite analysis of 624 sandstones, equal weights 
being taken. For the average limestone Clarke gives two determina- 
tions—one a composite analysis of 345 limestones and the other a 
composite analysis of 498 limestones used for building purposes. 
For the present purpose an average of the two composite analyses 
was taken, representing an average of 834 limestones. The several 
analyses selected are given in Table II. 
Most of the mass of the sedimentary rocks is derived from the 
crystalline rocks, but, in the process of redistribution, water and 
gases from the hydrosphere and atmosphere combine with the 
tF. W. Clarke, Bulletin No. 228, U.S. Geological Survey, 1904, p. 17. 
