FORMATION OF SEDIMENTARY ROCKS 247 
be drawn and the line of intersection projected onto the base. This 
line will indicate all combinations of the three rocks which contain 
a certain percentage of alumina. The point of intersection of the 
silica line and the alumina line on the cross-section paper at the base 
indicates what combination of the three rocks contains the same 
amounts of silica and alumina as a given fourth rock. That is, we 
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have found out in what ratios XY, Y, and Z must be taken to give a 
combination having the same content of silica and alumina as a 
given fourth rock. In the same manner this process may be con- 
tinued to include the other rock-making elements, and, if the lines 
on the cross-section paper do not intersect at a common point, it is 
possible by inspection to determine the point which comes nearest 
to all of the lines, and thus we may determine what proportions of 
X, Y, and Z will make a combination which is as nearly like that of 
