J. D. Whitney on the Oi-es of Iron in the Azoic System. 41 



teristics of true veins, the great masses of ore now under consid- 

 eration are wholly wanting. Some of the least important of them 

 approach much nearer to segregated veins, and might with pro- 

 priety be classed with them, were they not developed on so large 

 a scale as to render it difficult to conceive of segregation as a 

 sufficient cause for their production. 



In the case of the most prominent masses of ore of these 

 regions there is but one hypothesis which will explain their vast 

 extent and peculiar character. They are simply parts of the 

 rocky crust of the earth, and, like other igneous rocks, have 

 been poured forth from the interior in the molten or plastic state. 

 No other origin can be assigned to the dome-shaped and conical 

 masses of Lake Superior and Missouri, or to the elongated ridges 

 of the first-named region. The Iron Mountain of Missouri forms 

 a flattened dome-shaped elevation, whose base covers a surface 

 of a little less than a square mile, and which rises to a height of 

 200 feet above the general level of the adjacent country. The 

 surface of the mountain, where bare of soil, is found to be cov- 

 ered with loose blocks of peroxyd of iron, without any ad- 

 mixture of rocky pebbles or fragments, which increase in size 

 in ascending to the summit, where large blocks of ore many 

 tons in weight lie scattered about, and piled upon each other. 

 It is a most singular fact, that the ore is nowhere seen in 

 place about the mountain, although the whole mass evidently 

 consists of nothing else. Near its base, an excavation sev- 

 enteen feet deep has been made, which exhibits nothing but 

 small, somewhat rounded fragments of ore closely compacted 

 together, without any other substance present except a little 

 red, ferruginous clay, which seems to have been formed by the 

 friction of the masses against each other. This feature in the 

 Iron Mountain is one of peculiar interest, and one which it 

 seems difficult to explain. Evidences of drift action in this re- 

 gion are exceedingly faint. The ore itself is one which seems 

 little likely to undergo decomposition from any exposure to 

 atmospheric changes. The blocks upon the summit, although 

 somewhat moss-grown, have their angles and edges but little 

 rounded. As a key to the origin of the ore, we find in close 

 proximity on the north a long elevation of a reddish porphyry 

 of unmistakably eruptive character, connected with the Iron 

 Mountain by a narrow ridge of a rock composed of iron ore and 

 feldspathic rock, showing that the porphyritic ridge and the ore- 

 mass must have originated at one and the same time, and in the 

 same way. 



The eruptive origin of the great Lake Superior ore-masses 

 seems also well sustained by the phenomena which they exhibit. 

 They alternate with trappean ridges whose eruptive origin cannot 

 be doubted, and which, themselves, contain so much magnetic 



SECOND SERIES, VOL. XXII, NO. 64. — JULY, 1856. 6 



