]^0. 7.] GENESIS OF IRON ORES. 431 



CambriciD series ; but the characters of the great mass of these 

 rocks are such as to lead to the conclusion that they constitute, 

 as maintained by Emmons, a more ancient series. 



To the Taconian rocks belong the peculiar magnetic iron ores 

 found at Reading, Cornwall, and Dillsburg, Penn,, which have 

 been by some geologists regarded as Mesozoic, but were by 

 Rogers assigned to the base of the Palaeozoic. To this same 

 series belong the limonites of the great Valley, which occur in 

 clays derived from the sub-aerial decay of the rocks. These, in 

 their unchanged condition, contain beds and masses of siderite 

 and pyrites, and the alteration of these in situ has given rise to 

 the limonites. In the formation of this from the siderite, or iron- 

 carbonate, it was pointed out by the speaker that there is a con- 

 traction of volume equal to about 20 per cent. ; to which is due 

 the cellular character of the limonites and their frequent occur- 

 rence in the form of geodes. 



These older rocks are not without traces of oro;anic life, havinir 

 yielded in the Appalachian Valley the original ScoUthits and 

 related markings, besides obscure Brackiopods ; and in Ontario, 

 besides similar Scolithus-like markings, a form apparently iden- 

 tical with the Eozoon of the more ancient gneisses. We may 

 hope to find in the Taconian series a fauna which shall help to 

 fill the wide interval that now divides that of the Eozoic rocks 

 from the Cambrian. We should seek in the study of strati- 

 graphical geology not the breaks dividing groups from each other, 

 so much as the beds of passage which serve to unite all these 

 groups in one great system, remembering that there is no local 

 hiatus which is not somewhere filled up by the continuous process 

 of nature. 



II. THE GENESIS OF CERTAIN IRON ORES. 



(Abstract of a paper read before the American Association for the 

 Advancement of Science at Boston, August 28, 1880.) 



Dr. Hunt began by considering the presence of iron, generally 

 in a ferrous condition, in mineral silicates in the crystalline 

 rocks, and its liberation therefrom by the sub-aerial decay of these, 

 as hydrous ferric oxide. This, as is well known, is, by the agency 

 of organic matter, again reduced to ferrous oxide, which is dis- 

 solved in natural waters by carbonic acid, from which solutions it 



