^'^^' 57'] ANNIVERSARY ABDRE5S OF THE PRESIDENT. Ixxiil 



pyroxenes), calc-magnesian silicates (monoclinic pyroxenes and 

 hornblende), silicates of magnesium and potassium or of iron and 

 potassium (biotite), calcium-silicate (anorthite), silicates of sodium 

 and calcium (plagioclase), sodium-silicates (nepheline and albite), 

 and lastly potassium-silicates (orthoclase) in conjunction with 

 quartz. 



In his important work on slags, Prof. Yogt has clearly established 

 the influence of the relative proportions of the bases to each other and 

 to silica in determining the nature of the compounds which separate 

 out. Thus in slags in which the ratio of bases to silica corresponds 

 approximately to that found in bisilicates, the ratio of CaO : MgO 

 determines the formation of such minerals as enstatite, augite, and 

 woUastonite. When the ratio of MgO-f FeO : CaO is greater than 

 2-44 : 1, enstatite forms ; when the same ratio is less than 1*4 : 1, 

 augite separates out, and continues to do so, until this ratio becomes 

 less than '35 : 1 ; with a still further diminution in the ratio of 

 magnesia to lime, woUastonite is formed. 



In slags having approximately the composition of monosilicates 

 the ratio of MgO-f MnO-hPeO : CaO determines the formation of 

 olivine or melilite. When the above ratio is greater than 1:1-1 

 (in slags with about 20 per cent, of alumina), olivine is formed ; 

 but when it is less than 1 : 1-25, melilite is produced. 



The general conclusion arrived at as a result of the work of Vogt, 

 Lagorio, and others, is that mass-action and the affinities of the 

 bases to each other and to silica are the two factors of primary 

 importance in determining the molecular grouping, so long as the 

 pressure remains constant. The action of alumina may be especially 

 referred to, as illustrating the influence of the mutual affinities of 

 the so-called bases. In the sorting of partners in accordance with 

 the law of mass-action, this substance, when present in sufficient 

 quantity, practically takes the whole of the alkalies and as much 

 of the lime as is necessary to make felspathoid molecules. So 

 marked is this action that M. Michel Levy and M. Osann, in calcula- 

 ting the results of analyses, combine the whole of the alumina with 

 the alkalies, when the latter are present in sufficient quantity, and 

 associate any excess of alumina with lime in the form of felspathoid 

 molecules. It is only in those rocks that contain an abnormal per- 

 centage of alkalies that minerals like gegirine and riebeckite occur. 



This controlling influence of alumina, which has also been 

 emphasized by Prof. Iddings, has the most far-reaching eff'ects in 

 determining petrographical species. It is as if there were a kind of 



