834 A TREATISE ON METAMORPHISM. 



ACTIXOLITE-MAGJiETITE-CJlJARTZ ROCKS AXD GRtXERITE-MAGAETITE-qlARTZ ROCKS. 



The actinolite-magnetite-quartz rocks consist of interlocking actinolite, 

 magnetite, and quartz, with or without other iron oxides, and accessories of 

 various kinds. (PI. VII, D.) The griinerite-rnagnetite-quartz rocks consist 

 of interlocking griinerite, magnetite, and quartz, with or without other iron 

 oxides, and accessories of various kinds. In a given case the magnetite or 

 actinolite or griinerite may drop out, or nearly so. Where the magnetite 

 becomes subordinate the rocks are actinolite-quartz rocks and griinerite- 

 quartz rocks; where the actinolite or griinerite drops out the rocks become 

 magnetite-quartz rocks. 



The conditions under which these rocks form are those of the zone of 

 anamorphism. In many cases the actinolite and griinerite have a random 

 or radial fibrous arrangement to their bands, but in other cases they are 

 arranged with their longer axes in a common direction, with a marked ten- 

 dency toward crystallographic orientation. Where the parallel arrange- 

 ment of the amphibole occurs the quartz and magnetite also have a tendency 

 to have their longer dimensions in the same direction as that of the actinolite 

 and magnetite. Where this arrangement is marked — and this is very com- 

 mon — the term gneiss is properly applicable to the rocks; and thus we may 

 have actinolite-quartz-gneiss, magnetite-quartz-gneiss, etc. The amount of 

 parallel arrangement of the particles is a measure of the amount of orogenic 

 movement during the process of recrystallization. Where there is little 

 tendency to parallelism the rocks were probably crystallized by metasoma- 

 tism under mass-static conditions. Where the arrangement is well devel- 

 oped the recrystallization occurred during powerful orogenic movement. 



This relation is well illustrated by 'the Marquette district of Michigan. 

 In the central part of the district the folding is only moderately close, and 

 there the parallel arrangement of the mineral particles is very imperfect. 

 At the west end of the district the folding is of the closest and most intense 

 character, and there are beautiful coarse actinolitic and griineritic gneisses, 

 many of the mineral particles of which are arranged with almost perfect 

 parallelism. 



The most important change of the iron carbonate is to a silicate. 

 Where the carbonate is nearly pure siderite, griinerite is produced, accord- 

 ing to the following reaction: 



FeC0 3 +Si0 2 = FeSi0 3 +C0 2 , 



