THE AVERAGE IGNEOUS ROCK 773 



owing to the segregation of certain constituents, such as the mineral 

 matter in the sea and certain mineral deposits not included in the 

 sedimentary rocks. The solution of this problem was attempted 

 by the writer and the results published in this journal/ 



The former problem may be briefly stated as follows: given, 

 the average composition of the igneous rocks and of the shales, 

 sandstones, and limestones; to find the relative abundance of 

 these three important types of sediments. The present problem, 

 while somewhat similar, differs materially, and may be stated as 

 follows: assuming that the average igneous rock may be closely 

 approximated in composition by some combination of average 

 granite and average basalt, what ratio of granite to basalt best 

 explains the composition of the sediments as expressed by average 

 analyses of the shales, sandstones, and limestones, respectively; 

 and simultaneously with this solution, what ratio of shale, sandstone, 

 and limestone would result from the redistribution of this com- 

 bination of granite and basalt? The above might be stated in 

 the form of an equation as follows : 



X granite+>' basalt = shale+& sandstone+c limestone, given 

 average analyses of granite, basalt, shale, sandstone, and lime- 

 stone to solve for x, y, a, b, c. 



Note that no assumption is made as to the relative abundance of shale, 

 sandstone, and limestone, and that the solution of the problem is based 

 in no way on the results previously obtained^ or on any assumption 

 as to the average composition of the igneous rocks. The data used 

 in this problem are given in Table I. The average granite and 

 average basalt, columns i and 2, are those compiled by Dr. Daly.^ 

 The average analyses of the shales, sandstones, and limestones, 

 respectively, are those compiled by Clarke.'* Since the water and 

 carbon dioxide of the sediments are for the most part derived from 

 the hydrosphere and atmosphere, it is necessary in this problem 



^W. J. Mead, "Redistribution of Elements in the Formation of Sedimentary 

 Rocks," Jour. GeoL, XV (1907), 238-56. 



2 W. J. Mead, op. cit. 



3 R. A. Daty, "Average Chemical Compositions of Igneous-Rock Types," Proc. 

 Anier. Acad. Arts and Sciences, XLV (1910), 211-40. 



4 F. W. Clarke, op. cit., p. 28. 



