FOYAITE-IJOLITE SERIES OF MAGNET COVE 



659 



Of the Magnet Cove rocks this resembles most that of arkite 

 (IV), especially as regards Al^Og, (FeO), MgO, and Na^O, 

 though it is distinctly higher in SiOg and CaO and lower in 

 K3O. Referring it to Diagram 3, its position established by 

 means of SiO^ is shown at X, and the points where this vertical 

 is cut by the oxide curves are the "molecular ratios calc." of 

 the table. The small crosses along the vertical indicate the 

 positions of the various oxides as found. They can be identified 

 by the values in the table. 



It will be observed by reference to the diagram or to the last 

 two columns of the table, that in the case of oxides whose curves 

 are approximately straight lines, as Al^Og and Na^O, the found 

 and calculated values coincide, while in the case of oxides yield- 

 ing decided curves the value found is below that calculated. 



This is in accordance with the demonstration of Harker' 

 that if a series be linear the admixture of two or more members 

 will produce a rock having the composition of a possible 

 member of the series, while in a curvilinear series the mixture 

 will not correspond to a possible member. 



Another method for arriving at the composition of the 

 magma as a whole would seem to be furnished by the deter- 

 mination of the mean point of each of the curves, thus giving 

 the average composition. If the equations of the various curves 

 were known, these could be calculated mathematically. But for 

 practical purposes it can be done by determining, for each 



' Harker, Jour. Geol., Vol. VIII, p, 394, 1900. 



