FORMATION OF SEDIMENTARY ROCKS 243 
METHODS USED 
The problem now is to determine in what proportions the fore- 
going analyses of shales, sandstone, and limestone (Table IIT) should 
be combined so as to give an average analysis as nearly like that of 
the average crystalline rock as possible. ‘Cut and try” methods 
i a 
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are both tedious and unsatisfactory. A method of solution has been 
devised by means of which definite results may be obtained and the 
general conditions of the problem observed in perspective. 
It will perhaps make an explanation of the method of solution 
clearer if the general principles are first applied to the simpler case 
of a combination of two analyses. Suppose, for example, it is desired 
to find out what proportions must be taken of two rocks, A and B, 
having respectively 30 and 70 per cent. of silica, to yield a combina- 
tion that shall contain 4o per cent. of silica. Vertical distances in 
