232 THE CRYSTAL FALLS IKON-BEAKING DISTRICT. 



The altered character of the rock is readily seen in the large content 

 of water. It is, nevertheless, not so marked as to render the analysis use- 

 less for purposes of determination. 



The character of the plagioclase feldspar is clearly indicated by the 

 relatively hig'h percentage of lime. This higli content in lime and the large 

 amount of alkalies present, 7.18 per cent, clearly show its relationship to the 

 diorite family. The content in potash feldspar and the possible derivation 

 of the rock from a granitic magma is shown by the high content in potash. 

 Possibly a considerable amount of the potash, with the greater part of the 

 magnesia, should be deducted for the biotite which is so abundant. 



This rock is one which it is somewhat difficult to place definitely in 

 the existing division of rock families. The large amount of lime and the 

 relatively low percentage of alkalies prevent the placing of the rock with 

 the syenites. On the whole it approaches close to the monzonite group 

 according to the chemical composition of the group given by Brogger. 

 But it differs from this in that the lime (3.92 per cent) is too low to bring 

 the rock within his limits (4.52 to 10.12 per cent).^ However, if we con- 

 sider the total of the alkaline earths (7.65 per cent) in the rock under dis- 

 cussion, we find that it comes well within Brogger's range (6.05 to 17.52 

 per cent) for a total of magnesia and lime. Moreover, the alkali total (7.19 

 per cent) is about right to warrant its classification in the monzonite grouj) 

 as a representative of the type of biotite-monzonite. 



On comparing the analysis with that of trne normal diorites, we find 

 that the relative proportions of the alkalies are abnormal. The lime con- 

 tent is also too low for rocks of this character, and the magnesia is too high. 



The above considerations seem to make clear the relationship of the 

 rock to the monzonites and diorites. However, jt is so intimately asso- 

 ciated with and so evidently but a facies of the tonalite which is the domi- 

 nant type Avhere this rock occurs, that it is considered to be more closely 

 related to the lime-soda feldspar rocks, in which the orthoclase is but acces- 

 sory, than to the monzonite family of orthoclase-plagioclase rocks. It is 

 therefore considered to be a raica-diorite. 



' Op. cit., Part II, p. 51. 



