Alnoitic Rocks at Isle Cadieux, Que. 27 



Replacing or Reaction Relation of the Minerals of the Rock. 



The use of the word replacing in the foregoing discus- 

 sion and the division of the minerals of the rock into the 

 two classes, original and replacing, should not mislead the 

 reader into believing that the two sets of minerals are 

 considered to have formed under markedly different con- 

 ditions separated by an abrupt change. The chrysolite 

 and augite may very well have formed from a somewhat 

 alkalic liquid, but their separation, with decreasing tem- 

 perature, rendered the liquid so much more alkalic that 

 finally they became unstable in contact with such liquid 

 and were replaced by melilite and biotite with monticellite 

 as an intermediate step. It seems necessary to suppose, 

 however, that the crystals of chrysolite and augite had 

 accumulated locally and that there was only a relatively 

 small amount of alkalic liquid, so that the composition of 

 the mass as a whole was dominated by the richness 

 in chrysolite and augite and the new minerals formed 

 were determined mainly by this factor. In facies of the 

 rock where chrysolite and augite had not accumulated 

 in such large amounts the composition of the whole mass 

 was characterized by the greater richness in the alkalic 

 liquid and the new minerals formed from the augite and 

 chrysolite are alkalic pyroxenes, amphiboles and mica, 

 changes which have been described by those who have 

 studied the essexites and other rocks of this particular 

 province. 19 

 • The essexites, alkalic pyroxenites and other ultrabasic 

 rocks of this province, that have received a host of locality 

 names, have considerable interest in the present connec- 

 tion. The experimental results show that the fields of 

 the basic silicates, olivine, melilite, etc., extend over 

 beyond the join nephelite-diopside (metasilicate). The 

 fields of the metasilicates themselves are, of course, 

 crowded over in front of them to still more siliceous 

 mixtures^ The reaction relations of the alnoite minerals 

 indicate similar conditions in the natural magmas. The 

 suggestion is strong, therefore, that some of the basic 

 rocks of this province, in so far as they consist of augitic 

 pyroxene with some nephelite, could not have formed 

 from liquids of their own composition but must have 



18 In particular see G. A. Young, The Geology and Petrography of Mt. 

 Yamaska, Geol. Surv. Can. 16, p. 25H, 1906. 



