28o PENTTI ESKOLA 



the series are pyroxene and amphibole. With increasing acidity 

 biotite and muscovite appear successively. 



The occurrence and composition of clinopyroxene does not 

 always follow this rule, and for a very obvious reason : its occurrence 

 is due to the assimilation of limestone, which is dolomitic, and low 

 in iron. Therefore, when it reacts with the highly siliceous gneiss 

 magma, there is formed pyroxene and, also, hornblende and biotite, 

 low in iron, provided differentiation by crystallization has not 

 changed the composition of the magma after assimilation. In 

 many cases a differentiation happened, however, and therefore the 

 mafic minerals of very acid clinopyroxene gneisses not immediately 

 connected with limestone usually show a high Fe-quotient. 



Another interesting question is this : How are magnesia and the 

 iron oxides distributed between the different ferromagnesian 

 minerals ? 



As appears from the table on page 279, there are definite 

 relations between the pyroxenes, amphiboles, and biotites, in this 

 respect. Biotite is regularly the richest in iron and clinopyroxene 

 the poorest. The figures do not show any constant relations 

 between the Fe-quotients, however. This is only what may be 

 expected. From theoretical considerations the distribution of 

 elements in different isomorphous series in a rock is evidently a very 

 complicated process, depending upon the composition of the solu- 

 tions from which the minerals crystallized and, also, on the tempera- 

 ture and pressure conditions during crystallization. In rocks of 

 identical bulk composition these relations may be expected to vary 

 with the physical conditions. It may be hoped that, when once 

 they are better understood, we shall be able to draw from them 

 certain conclusions as to the conditions under which the rocks 

 originated. 



The present writer has formerly studied the almandite-pyrope 

 garnets of different rocks from this standpoint and has arrived at 

 the conclusion that, in rocks of similar composition, those formed 

 under highest pressures have garnets richest in the magnesia 

 compound.^ 



'P. Eskola, "The Mineral Fades of Rocks," Norsk geologisk Tidskrift, Vol. VI 

 (1920), p. 172. 



