9 6 J. P. ID DINGS 



The results of a study of the chemical composition of all kinds of 

 igneous rocks.- — -In attempting a study of the chemical composi- 

 tion of all kinds of igneous rocks it was found necessary at the 

 outset to devise some means by which the variations in the 

 chemical analyses could be compared with one another graphic- 

 ally or by tabulation. It was also thought desirable to consider 

 the chemical composition in its relations to the mineral compo- 

 sition, as far as possible. Further it was necessary to reduce 

 the number of variable quantities to be treated in any one 

 scheme. And for convenience and economy of labor the oxides 

 of the metals were employed instead of the metals themselves, 

 in all cases their molecular proportions 1 being used and not 

 their percentages by weight. 



In order to reach a basis for the correlation of the minerals 

 and the chemical compositions of the rocks, the minerals may be 

 divided into two groups, one embracing quartz, feldspars and 

 the feldspathoid minerals : leucite, nephelite, sodalite, and anal- 

 cite ; all except quartz containing aluminium and the alkali 

 metals or calcium, and being free from iron and magnesium. 

 The other group contains the amphiboles, pyroxenes, micas, 

 olivine, magnetite ; all excepting magnetite containing magne- 

 sium as well as iron. Muscovite may be classed with the first 

 group, but grades into biotite and may also be classed with the 

 second group. The first group includes orthosilicates of alu- 

 minium and sodium, metasilicates of aluminium, sodium, and 

 potassium, polysilicates of aluminium, sodium, and potassium, 

 and free silica. In each case the ratio between alumina and the 

 alkalis is a constant i: i; except in the sodalites, in which soda 

 is somewhat in excess of alumina. Calcium combines with alu- 

 minium in an orthosilicate molecule in which the ratio between the 

 alumina and lime is also i:i. This anorthite molecule combines 

 with albite molecules so as to form a continuous series of com- 

 pounds from orthosilicate to polysilicate. A consideration of the 

 occurrence of these minerals in igneous rocks shows that quartz 



1 Found by dividing the proportionate weights of each oxide by its molecular 

 weight. 



