FORMATION OF SEDIMENTARY ROCKS 245 
uprights correspond to the three rocks, X, Y, and Z, and are lettered 
accordingly. Vertical distances represent percentages of the elements 
as in Fig. 1. Points on the uprights representing percentages of 
silica are connected by a plane, instead of a line as was done in the 
case of the combination of two rocks. The ordinates of this plane 
represent any percentages of silica which may result from any pos- 
sible combination of the three rocks. To ascertain what points in the 
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plane represent 25 per cent. of silica, a plane is passed parallel to 
the base at a height of 25 units, and the line of intersection of the 
two planes contains all the points in the first plane which represent 
the desired 25 per cent. of silica. In the case of a combination of two 
rocks, as illustrated in Fig. 1, the ratios in which the two rocks 
entered into the desired combination was determined by project- 
ing the point of intersection of the two lines onto the line at the 
base. In the case of a combination of three rocks, we have a trian- 
