ORIGIN OF ACTINOL1TIC AND GRUNERITIC MARBLES. 833 



shales were subjected to profound mass-mechanical action and then were 

 transformed to jaspilites. Simultaneously with the process of jaspilitization 

 of the upper part of the iron-bearing- formation the basal conglomerate of 

 the Upper Marquette series, largely composed of the weathered material 

 of the iron-bearing formation, was transformed to a jasper-conglomerate. 

 It has been noted that the formation of the ferruginous shales and ferrugi- 

 nous cherts required four chemical transformations. Where jaspilites occur 

 a fifth chemical change has taken place — dehydration. 



ACTINOLITIC AXD GBUNERITIC MARBLES. 



In the zone of anamorphism the alterations of the iron-bearing carbon- 

 ates are entirely different in character from those in the zone of katamorphism. 

 In the lower zone the iron carbonates go through a set of transformations 

 which are analogous to those of limestones under similar conditions. Fur- 

 thermore, magnetite forms. Therefore the rocks which are first produced 

 are actinolitic or griineritic magnetitic marbles. In these rocks the chief 

 constituents are dolomite, magnetite, and actinolite or griinerite, often also 

 quartz. 



The development of the dolomite is the same as in the marbles. By 

 the silication of the carbonate either actinolite or griinerite is produced, 

 with diminution of volume— the former in case the bases present are calcium, 

 magnesium, and iron, and the latter in case the carbonate was originally 

 siderite or the original rock was greenalite (hydrous silicate of iron). 

 Where some iron sulphide or oxygen is available a part of the ferrous 

 iron may change to magnetite; and rarely a very little hematite may form. 

 Very frequently there is greater or less rearrangement and concentration of 

 the silica, so that bands or nodules of this material are formed. If the 

 metamorphism takes place under mass-static conditions, there is little 

 tendency for parallel orientation of the actinolite and griinerite. In propor- 

 tion as the alteration occurs under mass-mechanical conditions there is a 

 tendency for parallel orientation of the developing amphibole. Since 

 actinolite-magnetite rocks or grunerite-magnetite rocks, the end products of 

 the alteration of the siderites, ferrodolomites, and greenalite rocks are much 

 more common than the intermediate rocks treated under this heading, the 

 reactions producing the silicates are more fully discussed under the next 

 heading. 



mon xlvii — 04 53 



