116 MESSES. J. K. DAKTNS AND J. J. H. TEALL ON THE 



The diagram facing this page expresses the molecular relations of 

 the different bases and silica according to the method employed by 

 Mr. Iddings.-^ 



Molecular proportions of silica are represented by the abscissae ; 

 those of the bases by ordinates. The scale for the ordinates is 

 three-fourths of that for the abscissae, and the distance between two 

 of the ordinates (lY. and Y.) is somewhat exaggerated. The diagram 

 enables us to follow the changes in the relative molecular proportions 

 more conveniently than the table of analyses. 



As the silica increases magnesia falls from a very high position 

 in the peridotites to almost nothing in the eurite-veins. The fall 

 is not, however, uninterrupted. IVo rocks very close together as 

 regards amount of silica (lY. and Y.) differ in a manner opposed to 

 the general law. 



Lime first rises and then falls. After the fall has set in, it acts 

 in sympathy with magnesia; a local rise occurring at the same 

 time in both constituents. Both ferrous and ferric iron fall on the 

 whole, but there is a local rise corresponding to the local rise in 

 magnesia. 



Alumina rises and then falls, but not to any marked extent. 

 There is a rapid local fall corresponding with the abnormal rise in 

 lime, iron, and magnesia. It is clear that the ferro-magnesian con- 

 stituents are, relatively to the felspathic constituents, more abundant 

 in lY. than in Y. 



Soda rises and then falls. Its maximum is considerably to the 

 right of that for lime. Potash rises throughout the series, except at 

 the point where magnesia rises. It is interesting to compare the 

 two alkalies. First the soda increases more rapidly than the potash ; 

 then the relations are reversed. Between Y. and lY. the potash 

 falls as the soda rises ; between lY. and YI. they rise together ; 

 and between YI. and YII. the potash rises as the soda falls.. 



It is of course quite easy to connect the variations in the chemical 

 constituents with the changes in mineralogical composition. Eirst 

 of all the ferro-magnesian constituents decrease as the felspathic 

 constituents increase, and then a change takes place in thq felspathic 

 constituents and free quartz is introduced. 



Anorthite-molecules decrease relatively to albite- and orthoclase- 

 molecules, and finally albite-molecules decrease relatively to ortho- 

 clase-molecules. 



5. Helatioi^s betwee]^ Mineralogical Composition^, Chemical 

 Composition, and Geological Age. 



Pield evidence has established the fact that the rocks rich in 

 ferro-magnesian constituents have preceded those poor in these con- 

 stituents, and that rocks in which anorthite-molecules enter largely 

 into the composition of the felspars have preceded those in which 



^ 'The Mineral Composition and Greological Occurrence of certain Igneous 

 Eocks of the Yellowstone National Park,' Bull. Ptiil. Soc. Washington, vol. xi. 

 pp. 191-220. 



