32 B. S. Butler — Pyrogenetic Epidote. 



equal abundance in various stages of alteration of the differ- 

 ent dikes. Where secondary epidote develops in the altera- 

 tion of biotite and feldspar, it is in minute grains and shows 

 no tendency to collect in large crystals. The very low lime 

 content of the rock would permit of the formation of but a 

 small amount of epidote, and it is difficult to conceive of con- 

 ditions of alteration that would cause all this to collect in a 

 few large crystals, if it had been originally disseminated 

 through the rock. In the freshest dikes the feldspars show 

 but slight alteration, and could not have furnished sufficient 

 CaO from this alteration to form the epidote. The enclosing 

 rock is extremely low in lime, and cannot be looked upon as 

 a source of this material for the formation of the epidote. 

 The dikes are near the center of a large intrusive mass and 

 therefore are probably not affected by formations surrounding 

 this large mass. 



Considering the dikes as the result of differentiation of the 

 magma represented by the main intrusive mass, it is seen that 

 there has been a decrease in Si0 2 with increase in most of the 

 remaining oxides ; the relative increase in lime is much greater 

 than in soda. From this it would naturally be expected that 

 the feldspar of the dikes would be distinctly more basic than 

 that of the main intrusive. The feldspar of the groundmass 

 in both rocks is too small for accurate determination, but so 

 far as can be judged by the phenocrysts, there is little differ- 

 ence in the composition of the feldspars in the main intrusive 

 and in the dikes. Assuming that the feldspars in the two 

 rocks are of the same composition, the excess of lime in the 

 dike rock may be considered as available for the formation of 

 epidote. 



In calculating the composition of the dike rock we may 

 assign enough CaO to combine with available P 2 5 to form 

 apatite, an amount equal to the total CaO present in the main 

 intrusive to form anorthite, and there is still remaining suffi- 

 cient to form 1*4:1 per cent of epidote of the composition 

 shown by the analysis. As the amount of feldspar in the 

 dikes is greater than that in the enclosing rock, it would 

 require slightly more CaO than is present in the main intru- 

 sive to form feldspar of the same composition. This would 

 reduce the amount of epidote slightly, but it would probably 

 still be above one per cent. It is difficult to estimate the per- 

 centage of epidote present in the scattered crystals, but it 

 certainly seems to correspond well with the amount roughly 

 calculated above. 



Washington, D. C. 



