FORMATION OF SEDIMENTARY ROCKS 255 
be in accord with the results obtained in the present investigation, 
which are based entirely on chemical evidence. This lack of accord- 
ance should not be interpreted as evidence that either of the ratios is 
incorrect. The figures obtained from field observations take into 
consideration only the present continental areas, and if it is assumed 
that they correctly represent the results of redistribution, it is also 
necessary to assume that the sedimentary formations at present on 
the continental areas constitute practically all of the redistributed 
material, or that sedimentation has been uniform over the earth’s 
entire surface, in which case an average geological column for the 
continents would represent average sedimentation. The ratio based 
entirely on chemical evidence is not necessarily limited by the assump- 
tions above mentioned, and for this reason it is believed that 80 per 
cent. shale, 11 per cent. sandstone, and g per cent. limestone is a 
better measure of the net results of redistribution than figures based 
on field evidence alone. 
SUMMARY AND CONCLUSIONS 
The sedimentary rocks are ultimately derived from the igneous 
and crystalline rocks. ‘The general average composition of the sedi- 
mentary rocks should equal the average composition of the igneous 
and crystalline rocks, when minor corrections are made for the 
materials not classed as sediments, but derived from the same source, 
such as the salts of the ocean and ore deposits. The three main 
classes of sediments constitute fully 97 per cent. of the end-products 
of the destruction of the igneous and crystalline rocks. It should 
be possible to combine these three great groups in such a manner as 
to make them yield an average composition similar to that of the 
parent rock. 
It has been the purpose of this investigation to determine the 
proportion which most nearly accomplishes this result. A quantita- 
tive method of solution has been devised which permits of ready and 
accurate solution of the problem. 
The result has been found to be 80 per cent. of shales, 11 per cent. 
of sandstones, and 9 per cent. of limestones. 
This proportion accords with that derived from other methods of 
