256 WARREN J. MEAD 
investigation in showing the striking dominance of shales in the sedi- 
mentary rocks. 
The best available average analyses may not be correct, but the 
probable error, determined by noting the variations in the data avail- 
able, is not so wide as to materially affect the result. It is believed 
that the proportion thus determined is entitled to more consideration 
than a proportion determined by measurement of field sections or of 
materials transported by rivers. If this proportion be not essentially 
correct, the best available average compositions vary essentially from 
the truth. 
The small discrepancies between the composition of the parent 
rocks and the resultant principal groups of sedimentary rocks are 
found to be still further reduced when the relatively minute amounts 
of the minor end-products of the destruction of the parent rocks are 
taken into consideration. 
The proportion of limestone determined (g per cent. of the prin- 
cipal sedimentary rocks) is smaller than that obtained by measure- 
ment of geological sections. If the apparent difference is a real one— 
in other words, if the data which have entered into the two methods 
of calculation are reasonably correct—it may be concluded that there 
has been a concentration of limestone in the continental area. Whether 
this concentration is simply one of present areal distribution, the 
fragmental equivalents of the limestone at the present time being 
largely under the oceanic area, or whether the limestone has been 
uniformly concentrated through geological time in the upper part of 
the lithosphere, as has been held by certain writers, remains to be 
proved. 
The work here outlined has been done in the metamorphic labora- 
tory of the University of Wisconsin. I am greatly indebted to Dr. 
C. K. Leith for direction and criticism. 
