JASPILITES AND FERRUGINOUS SHALES AND CHERTS. 829 



It is concluded from the foregoing that the main sources of the iron- 

 bearing carbonates are the iron-bearing silicates of various rocks, and 

 especially intermediate and basic igneous rocks which are porous. From 

 these, iron carbonate and other soluble iron salts are produced by the 

 reactions of the belt of weathering. They are transported to inclosed or 

 partly inclosed bodies of standing water by the underground solutions. 

 The material is there thrown down, mainly as ferric hydrate. By the 

 action of the organic matter upon the ferric iron it is reduced to ferrous 

 oxide and unites with the carbon dioxide simultaneously produced, or with 

 silicic acid, forming iron carbonate or iron silicate. So far as the iron is a 

 sulphate it is reduced wholly or in part to sulphide. 



In what manner the calcium and magnesium are thrown down with 

 the iron is uncertain. In the inclosed lagoons, which must have been of 

 exceptional character, as shown by the precipitation of the iron itself, it is 

 possible that calcium and magnesium might so accumulate as to be precipi- 

 tated chemically, and thus the carbonate be originally an iron-bearing 

 calcium-magnesium carbonate, in which the iron varies from a subordinate 

 to a dominant constituent. This suggestion is the more plausible because 

 where iron is abundantly precipitated as hydroxide and remains in that 

 form life is usually somewhat sparse. While this may be a partial explana- 

 tion of the precipitation of the calcium and magnesium, the calcium may 

 have been mainly thrown down through the instrumentality of life, in the 

 same manner as were the ordinary limestones, and the substitution of 

 magnesium for calcium may have occurred subsequently, precisely as in 

 the case of the limestones, thus forming the ferrodolomite. 



FERRUGINOUS SHALES, FERRUGINOUS CHERTS, AND JASPILITES. 



Iii the zone of katamorphism ferruginous shales or ferruginous cherts 

 are produced from the iron-bearing carbonates, and by modifications of 

 these in the zone of anamorphism jaspilites are formed. By ferruginous 

 shales are meant rocks which have a bedded structure and are composed 

 mainly of oxide of iron, but which include a greater or less quantity of 

 finely disseminated quartz. By ferruginous cherts are meant those rocks 

 which consist of two sets of bands, one of which is composed mainly of 

 iron oxide and the other mainly of chert. 



