METAMORPHISM OF IRON-BEARING CARBONATES. 839 



rock. In still other instances all three classes of minerals are abundantly 

 present, and we have actinolite-magnetite-quartz rocks or griinerite- 

 magnetite-quartz rocks. Each of these different combinations of minerals 

 may be of sufficient extent to constitute a member of a formation, or even 

 an entire formation. 



Which of the above rocks develops at a given place depends not only 

 upon the original composition of the rocks, but upon the nature of the 

 alteration. For instance, where in the original rock silica is subordinate 

 and nearly pure siderite abundant, a quartzose-mag-netite may develop, as 

 at various places in the Lake Superior region. Where the conditions are 

 such that the silicates form, the development of the actinolite or griinerite 

 uses up both the iron carbonate and the silica, and an actinolite rock or a 

 griinerite rock may be produced. Where silica was originally an abundant 

 constituent both magnetite and the silicates are likely to develop. Thus 

 we have various proportions of all the minerals, producing- the magnetite- 

 quartz rocks, the actinolite-magnetite-quartz rocks, the griinerite-magnetite- 

 quartz rocks, the actinolite-quartz rocks and the griinerite-quartz rocks. 



Usually a given formation, or member, does not show a perfectly 

 homogeneous arrangement of the mineral particles. The original sedi- 

 mentary rock is banded, and the different bands have different compositions. 

 Naturally the transformation of these bands produces different combinations 

 of minerals. Moreover, during- the recrystallization there is a tendency, 

 as explained in Chapter III (pp. 120-123), for minerals of the same kind to 

 segregate. Hence, in any of the above cases, where as a whole a certain 

 set of minerals are dominant within a rock, a single mineral, or two com- 

 bined, may be largely segregated in bands ; and in the alternate bands the 

 other minerals be largely segregated. Thus a banded rock, consisting- 

 mainly of magnetite and quartz, may have a banded appearance as the 

 result either of the segregation of the quartz and magnetite in separate 

 bands or, more commonly, the segregation of more quartz and less magnet- 

 ite in one band and less quartz and more magnetite in another band. In 

 a similar manner alternate bands may be made up of actinolite or griinerite 

 with quartz in various proportions, and of actinolite or griinerite with 

 magnetite in various proportions. In still other instances the banding mav 

 be due to the combining of actinolite or griinerite, magnetite, and quartz 

 in various proportions. In general, therefore, the alterations of the rock do 



