418 WEED AND PIRSSON — HIGHWOOD MOUNTAINS OF MONTANA. 





Rock analyses. 





Chief ox 



ides to 100. 



Molecules. 





A 



B 





Ai 



Bi 



A2 



B'^ 



Si02 .. 



. 56.45 



46.73 



SiO,.. 



.. 57.83 



48.36 



65.61 



49.27 



TiO^. . . 



. .29 



.78 



A1A-. 



. . 20.57 



10.40 



13.62 



6.27 



AlA-- 



. 20.08 



10.05 



FeO.. 



.. 5.72 



11.78 



5.39 



10.02 



FeA- 



. . 1.31 



3.53 



MgO.. 



.. .64 



10.01 



1.10 



15.28 



FeO... 



. 4.39 



8.20 



CaO . 



.. 2.19 



13.68 



2.65 



14.84 



MnO... 



, -. .09 



.28 



Na,0.. 



. . 5.75 



1.88 



6.33 



1.82 



MgO... 



. .63 



9.68 



K2O . 



, . 7.30 



3.89 



5.30 



2.50 



CaO... 



Na,,0.. 



. 2.14 

 .. 5.61 



13.22 

 1.81 















100.00 



100.00 



100.00 



100.00 



K2O... 



. 7.13 



3.76 













H2O... 



. 1.77 



1.24 













P2O5... 



. .13 



1.51 













CI 



. .43 



.18 















100.45 



100.97 





= C1. 



.10 



• .04 











• 



Total. . 



100.35 



100.93 





In the above table the analysis of the s^^enite by Melville is given under 

 A ; that of the shonkinite by Pirsson is repeated under B. For purposes 

 of more easy comparison they are repeated under A' and B\ with the 

 non-essential elements omitted, the ferric iron reduced to ferrous, and 

 the whole brought to 100. This at once brings out the most important 

 chemical characteristics of the shonkinite. its very high iron, lime and 

 magnesia, properties which show its difference from the typical syenites 

 and its approach to the basaltic and lamprophyre groups. In A'-^ and B' 

 are given the percentages of molecules in the rocks derived from the 

 oxygen ratios. The percentages by molecules gives in genera] a much 

 clearer idea of the chemical composition of a rock than that by weight, 

 because it shows more correctly its capacity for forming minerals. 



From the above table it is seen at once that the magnesia shows the 

 greatest differentiation, then the lime, and then iron. The relative pro- 

 portion of the alkalies to each other and to alumina is about the same 

 in each ; they vary some, it is true, but the variation is insignificant com- 

 pared with that of the bivalent oxides. The tendency of variation, then, 

 has been for the lime, iron and magnesia molecules toward the outer 

 cooling surface, while the alkalies and alumina have remained a constant, 

 or if we imagine the silica to remain a constant, they have moved in- 

 wardly. It is also clear that the bivalent oxides have not kept a nearly 

 constant ratio, for magnesia is much more concentrated than iron. 



Of course, this implies that the molten mass before intrusion into the 

 laccolite cavity was of uniform composition ; that one liquid mass of 

 one kind was not succeeded by another of different composition. The 



