11 N. L. Bowen — Genetic Features of 



classes of original and replacing minerals and pictures 

 the original minerals as they were before replacement 

 began, it becomes apparent that there could have been no 

 great amount of any material in much of the rock other 

 than augite and chrysolite. It is probable that there was 

 a moderate amount of interstitial liquid which gave oppor- 

 tunity for such uniform action at all points in the mass 

 and upon each individual mineral grain. It is probable, 

 too, that this interstitial liquid was itself of such a nature 

 as to enter into reaction, as the temperature changed, with 

 the crystals that had already separated from it, in such 

 a manner as to produce the new minerals actually found. 

 It is not probable, however, that the interstitial liquid 

 was of sufficient quantity to produce so much change. 

 Rather is it to be supposed that there was a movement 

 of liquid of the same kind through the interstices of the 

 mass, that the liquid reacted with the solid phases and 

 passed on (or was crowded out), carrying with it some 

 of the products of the reaction until finally the interstices 

 were filled up. 



However these details may fit the fact, it is certain that 

 a liquid permeated the rock and partly replaced augite 

 and chrysolite by monticellite, biotite and melilite. Now 

 it is believed that the seams of melilite-biotite rock 

 represent simply streaks along which the freedom of pas- 

 sage of liquid has been greater and where the replacement 

 is more advanced. In these monticellite is corroded by 

 biotite and melilite in much the same manner as are the 

 original minerals in the main mass. The monticellite 

 is, then, to be regarded as an intermediate step in the 

 replacement of augite and chrysolite by melilite and 

 biotite. 



Nature of the Reacting Liquid. 



The question naturally arises as to the nature of the 

 liquid (magma) which could produce results of the kind 

 noted. The formation of such minerals as monticellite 

 and melilite might on first thought be considered to 

 require a lime-rich liquid. On the other hand the for- 

 mation of biotite points definitely to an alkalic liquid 

 and it will be found that there are good reasons for 

 believing that an alkalic liquid is capable of producing 

 the lime-rich minerals as well. The alkalic liquid is, 



