﻿part 2] 



PICEITE-TESCHElSriTE SILL OF LUGAE. 



119 



relation to the ' maficity,' and may accordingly be used as abscissse 

 in a diagram, against which the other oxides may be plotted as 

 ordinates (see fig. 6). Just as in dealing with the mineralogical 

 variations, it has been found necessary and desirable to average 



10 15 20 25 30 3o 



PERCENTAGES OF McjO + FoO 



the chemical composition of the various rock-types for use in the 

 diagram. The averages thus arrived at are given in Table VIII. 

 In the calculation of these averages most of the chemical com- 

 positions calculated from the graphic analyses have been used, 

 omitting those which duplicate the actual chemical analyses made 



