ORIGIN OF SIDERITE, ANKERITE, AND PARANKERITE. 825 



belt of weathering are rather rich in iron; for instance, intermediate or basic 

 igneous rocks. If rock of this favorable composition were scoriaceous 

 volcanic rocks, the conditions would be exceptionally favorable to the pro- 

 duction of such salts by the process of carbonation of the silicates, the 

 partial reduction of the ferric iron, and its union with carbonic or sulphuric 

 acids. 



Such conditions are illustrated by the Lake Superior region, where the 

 iron for the iron-bearing formations was largely derived from basic volcanic 

 rocks. In that region in each of the important iron-bearing- districts below 

 the iron-bearing series are found greenstones, often ellipsoidal, in many 

 places porous and amygdaloidal, in many places schistose, all rich in iron. 

 Below the Mesabi, Penokee, Marquette, and Menominee series are great 

 masses of basic volcanic rocks. In the Penokee and Crystal Falls districts 

 basic volcanic outflows were contemporaneous with or immediately preceded 

 the deposition of the iron-bearing formation. In the Vermilion district of 

 Minnesota and in various districts of Canada, including the Michipicoten 

 district, the iron-bearing formations immediately overlie enormous masses of 

 ellipsoidal and often amygdaloidal basic volcanic formations. 



The ground waters take iron compounds abundantly into solution 

 wherever the rocks are of a composition and texture favorable to furnishing 

 iron. The waters take a longer or shorter journey before issuing at the 

 surface. The issuing waters, aside from the iron compounds, contain, among- 

 other abundant compounds, the calcium and magTiesium carbonates. Where 

 extensive iron- carbonate deposits have formed, it is probable that the sources 

 of supply were adjacent to shallow standing bodies of water, such as arms of 

 seas, lagoons, estuaries, etc. Under such circumstances the ground waters 

 usually issue at the lower places a short distance above or below the level 

 of the water. As has been explained, such bodies of water are generally 

 rich in vegetation. In these bodies the iron salts are precipitated. 



As to the form in which the iron salts enter the seas, we can judge only 

 by analogy, but if the present be a guide to the past, the iron was chiefly as 

 a carbonate and to a subordinate extent as a sulphate, although it might 

 have been in part in the form of other salts. When the iron salts reach the 

 lagoon, they are precipitated under favorable conditions as ferric hydrate or 



