842 A TREATISE ON METAMORPHISM. 



altered. The ferruginous shales and ferruginous cherts, products of the 

 zone of katamorphism, when transferred to the zone of anamorphism are 

 transformed to jaspilites. The actinolitic and griineritic rocks transferred 

 to the zone of katamorphism are altered to ferruginous shales and ferrugi- 

 nous cherts, but this change takes place so very slowly that these products 

 have not been thus produced on a very large scale. 



OXIDE ORDER. 



IRON-OXIDE FAMILY. 



The iron-oxide family comprises a number of oxides — anhydrous and 

 hydrous. Of these limonite, hematite, and magnetite occur in sufficient 

 masses to entitle them to consideration as rocks. Limonite is hydrated 

 ferric oxide (2Fe 2 3 .3H,0). Hematite is ferric oxide (Fe 2 3 ). Magnetite 

 is ferrosoferric oxide (Fe 3 4 ). Many of the larger formations are inter- 

 mediate between limonite and hematite. Some are partly hematite and 

 partly magnetite. Usually subordinate minerals are associated with the 

 iron oxides. Of these, quartz is almost universal. But all the carbonates, 

 many of the hydroxides and silicates, and some of the sulphides are often 

 associated. The carbonates and hydroxides other than of iron are apt to be 

 associated with the limonites, and less commonly with the hematites. The 

 silicates and sulphides are usually associated with the magnetites, and often 

 with the specular hematites. 



LIMONITE. 



Limonite may compose considerable rock deposits. Ordinarily such 

 deposits are not very thick, although they may have considerable extent. 

 The deposits are commonly of very irregular thickness, or bunchy, being 

 locallv perhaps 3, 6, or 10 meters thick, and within a short distance almost 

 disappearing. 



Recent deposits of limonite are extensive. These occur in marshes 

 and bogs. They have been worked to a greater or less extent as iron ores 

 throughout the civilized world. Probably the most extensive deposits of 

 limonite known are those of the Lower Silurian, which have been worked 

 for ore in many countries. 



The Silurian limonites and the limonites of many other horizons are 

 largely replacement and concentration products of limestones, being there- 

 fore a result of metamorphism. In some districts the limestones originally 



