SOME EXAMPLES OF ROCK VARIATION 389 



On comparing the analysis with that of a normal diorite, 

 we find the relative proportions of the alkalies are abnormal. 

 Also the lime content is too low for rocks of this character ; 

 and, again, the magnesia is too high. From the above consid- 

 erations it seems clear that the rock is related to the monzonites 

 and diorites. However, it is so intimately associated with, and 

 so evidently a facies of the tonalite, which is the dominant type 

 where the mica-diorite occurs, that it is considered to be more 

 closely related to the lime-soda-feldspar rocks in which the 

 orthoclase is but accessory, than to the monzonite family of 

 orthoclase-plagioclase rocks. It is, therefore, considered to be 

 a mica-diorite. It has already been remarked that while for 

 normal diorites the lime is too low, the magnesia is correspond- 

 ingly too high. May we not with right consider this as indi- 

 cating a relationship to the more basic rocks gabbros, in which 

 magnesia forms a very important constituent and with which it 

 is so intimately associated in the field ? As against this interpre- 

 tation, however, we have a very high percentage of alkalies and 

 moderately high percentage of silica, which certainly warrant 

 the exclusion of this rock from the gabbro family. 



When we turn to a consideration of the gabbro-norites as 

 represented by analyses Nos. 2 and 3, it is at once clear that if 

 we accept, as has been done in the preceding pages, Brogger's 1 

 characterization of the diorite and gabbro families, that these 

 rocks could not be included with the diorites as respectively 

 normal diorite and bronzite-diorite, but must, from their abnor- 

 mally low silica and alkali content and high alumina, lime, and 

 magnesia content, be placed with the gabbros. Especially note- 

 worthy in analysis No. 2 is the high percentage of alumina 

 present. Normally, large alkali content accompanies high per- 

 centage of alumina. A reference to the alkalies shows this not 

 to be true in this instance. * 



Analysis No. 4 is not to be taken as representing the most 

 basic variety of peridotite in this district. From this alone the 

 statement that the variations extend to the ultrabasic rocks 



'Op. cit., Part II, pp. 35, 39. 



