Rbid.] 



The Igneous Rocks near Pajaro. 



189 



In this triangulation method an equilateral triangle is divided 

 each way into one hundred parts, each division being thus taken 

 as a graphical representation of one per cent. The triangle 

 is not shown in its entirety, as all the plots fall in the lower 

 right-hand corner. The Si02 is set off first from the left leg 

 of the triangle and marked on the right leg, making the apex 

 of a new triangle whose altitude is a measure of the acidity or 

 basicity of the rock. The second part of the plot is located by 

 the use of the alumina and total iron percentages, as follows: 

 The alumina content is set off to the right from the left leg of 

 the new triangle, and the total iron is set off from the right leg 

 to the left. The intersection of these two lines gives the second 

 point of the plot. By connecting the two points found by a line 

 the first segment of the plot is found. The second and third 

 points are found in a similar manner, using the lime-magnesia 

 and soda-potassa percentages, respectively. By joining these 

 points by straight lines the plot is completed. 



From each point found as above a triangle may be drawn, 

 whose altitude is a measure of the content of the oxides whose 

 lines fall within it. For instance, from the silica point a triangle 

 may be drawn whose altitude measures the total content of the 

 oxide in the rock, and inversely measures the acidity. From 

 the second, the alumina-iron point, a triangle may be drawn 

 whose altitude expresses the content of the lime, magnesia, soda, 

 and potash, and a comparison of this with the first altitude gives 

 the relative proportion of alumina and iron to silica. Similarly 

 the other points serve as the apices of triangles whose altitudes 

 express relative proportions of the different oxides. Also the 

 length of the different segments is a similar measure, and the 

 direction of each indicates the relative abundance of the two 

 oxides whose content determine it. 



Each plot thus consists of four points and three segments, 

 which may or may not be calculated on a water-free basis. For 

 the series of igneous rocks, or better, rock magmas, the plots of 

 the basic occupy the left or center of the triangle, while the acid 

 occupy the right. The method is yet not all to be desired, nor 

 fully worked out in all its possibilities, still the plots serve 

 well to bring out the likenesses and differences of the rocks cited. 



