J. D. Whitney on the Ores of Iron in the Azoic System. 39 



In illustration of these views, we have briefly described some 

 of the great ferriferous districts of the world, and particularly 

 those of Lake Superior, Scandinavia, Missouri, and Northern 

 New York, all of which exhibit a most marked analogy with 

 each other, both in regard to the mode of occurrence and the 

 geological position of the ores. The two last-named regions, 

 however, not having been thoroughly examined by us in person, 

 we were obliged to content ourselves with information obtained 

 from others, in making a comparison of their most striking 

 features. 



Strongly impressed with the interest attaching to this subject, 

 I availed myself of the first opportunity, after the publication of 

 our Eeport, to visit the iron regions of Missouri and Northern 

 New York, from the last-named of which I have just returned, 

 after a careful examination of the most important localities where 

 ore is now mined in that district. While it is intended to take 

 another opportunity for giving a minute and detailed account 

 of this region, I may be permitted to recapitulate here the prin- 

 cipal points maintained by Mr. Foster and myself, to the gen- 

 eral correctness of which my more recent explorations have 

 furnished me with additional evidence. 



We maintain therefore, — 



1. That deposits of the ores of iron exist in various parts of 

 the world, which in extent and magnitude are so extraordinary 

 as to form a class by themselves. The iron regions mentioned 

 above offer the most striking examples of the deposits now re- 

 ferred to. 



2. That the ores thus occurring have the same general charac- 

 ter, both mineralogically and in their mode of occurrence, or 

 their relations of position to the adjacent rocks. 



3. That these deposits all belong to one geological position, 

 and are characteristic of it. 



The extent of the workable deposits of the ores of the useful 

 metals is usually quite limited. Most of the veins which are 

 wrought in mines throughout the world are but a few feet in 

 width, often not more than a few inches. This is true of the ores 

 occurring in veins. In sedimentary metalliferous deposits, such 

 as those of the ores of iron in the carboniferous, the horizontal 

 extent is often very considerable ; but the vertical range is so 

 limited, that the most extensive basins may be in time exhausted, 

 when worked on so extensive a scale as is the case in some of the 

 celebrated iron districts of Great Britain. The deposits of iron 

 in the azoic, however, are many of them developed on a scale 

 of such magnitude, that the term " mountain masses" may be 

 applied to them without exaggeration, while, from the very nature 

 of their occurrence, they must extend indefinitely downwards, 

 and cannot be exhausted. Thus the great iron mountain of 



