630 /. E. L. VOGT 



tion of a medium mixture principally of olivine (3.22) and anorthite 

 (2,765) which in most cases replace it in the deep-seated rocks. 



In all of these three cases we thus observe the fact that a com- 

 bination of minerals, respectively a single mineral (leucite, melilite) 

 in the effusives are replaced in the deep-seated rocks by a single 

 mineral (garnet), respectively a combination of minerals, charac- 

 terized by increased density. 



This recalls the well known "volume law" valid for dynamo- 

 metamorphism, viz., that various minerals under high pressure 

 form new combinations with the minimum total volume — a law 

 whose physicochemical cause, moreover, is not exactly elucidated. 



Since the above section, in the main, was written, in the winter 

 1919-20, a treatise on "The Mineral Facies of Rocks," by P. 

 Eskola,^ was published, wherein he tries to establish some (five) 

 facies for the mineral combinations depending on the pressure not 

 only for the metamorphic, but also for the igneous rocks. Eskola 

 classes with the last, the facies of the igneous rocks formed under 

 the highest pressure, among others those especially investigated by 

 him, namely garnet-rich (with up to 75 mol. per cent pyrop- 

 component MgjAla SiOjOiz) eclogites, which "occup);^ a volume about 

 15 per cent smaller than that of the corresponding gabbros." 

 Eskola thus maintains the same views as those I have pointed out 

 above. I call attention to this treatise, in which are given some new 

 instances of the interdependence of the formation of certain mineral 

 combination in the igneous rocks upon the pressure. Further on 

 Eskola treats of the interdependence of the metamorphic new- 

 formations upon the pressure, which matter I do not touch in this 

 treatise. 



{To he continued] 



^ Norsk geol. Tidsskrift, Vol. VI (1920). 



