IRON-BEARING CARBONATES. 823 



such localities the formations are usually not extensive. These deposits 

 are well illustrated by the kalk-silikat-hornfelse of the Harz Mountains, 

 described by Lossen and Rosenbusch" 



GENERAL STATEMENTS. 



The processes of crystallization, dolomitization, silicific'ation, and silica- 

 tion of the limestones and the resultant rocks have been separately described 

 as if each occurred alone. As a matter of fact, in the field all the processes 

 may occur together in various proportions, and hence we may have all 

 gradations between limestone and dolomite, between limestone and marble, 

 between pure limestones, dolomites, and marbles and siliceous limestones, 

 dolomites, and marbles, and between these rocks and those in which the 

 process of silicification is complete. Finally, we may have all gradations 

 between the various forms above mentioned and those in which the sili- 

 cates have developed in subordinate, important, or dominant amounts. 

 Therefore, within the limestone series there are complete gradations between 

 the various rocks of the series. 



IRON-BEARING CARBONATE FAMILY AND METAMORPHOSED EQUIVALENTS. 



The iron-bearing carbonate family and metamorphosed equivalents 

 comprise siderite, ankerite, and parankerite; ferruginous shales, ferruginous 

 cherts, and jaspilites; actinolitic and griineritic marbles; and actinolitic 

 quartz rocks and griineritic quartz rocks. 



SIDERITE, ANKERITE, AND PARANKERITE. 



The original forms of the iron-bearing carbonates include siderite, 

 ankerite, parankerite, and gradations between them. (PI. VII, A.) With 

 these carbonates,' as with the less ferriferous carbonates, chert is an almost 

 universal associate. It varies in amount from a minute quantity to an 

 important amount, and occasionally to dominance. -The alterations of the 

 above carbonate compounds as minerals have been considered on pages 

 242-245. As rock masses they may be considered under the terms siderite 

 and ferrodolomite, including under the latter term all gradations between 

 siderite on the one hand and dolomite on the other. 



The iron-bearing carbonates occur mainly in layers in the stratified 

 rocks, and are therefore of sedimentary origin. 



"Rosenbusch, H., Physiographie der Mineralien, Stuttgart, 1896, vol. 2, p. 97. 



