246 WARREN J. MEAD 
cular surface at the base instead of a line. For this triangular sur- 
face a piece of triangular cross-section paper is used, like the one 
shown in Fig. 3. The three corners represent respectively 100 per 
cent. of X, 100 per cent. of Y, and 100 per cent.of Z. Any point in 
the triangle represents some combination of X, Y, and Z, and the 
percentage of each in the combination is measured by the distance 
ldiey a 
between the point and the side opposite. To illustrate: the point P 
represents a combination made up of 4o per cent. X, 50 per cent. 
Y, and ro percent. Z. The point O in the center of the triangle 
represents a combination containing 33% per cent. of each of the three. 
If the line of intersection of the two planes in Fig. 2 be projected 
vertically downward onto the base, it will represent all possible com- 
binations of X, Y, and Z which contain 25 per cent. silica. By the 
same process the planes for the alumina content of the rocks may 
