. PROCEEDI.NGS OF THE POLYTECHNIC ASSOCIATION. 493 



geological date, are the magnetic oxides which are found in the 

 azoic rocks. Such ores are found in masses called "Iron moun- 

 tains" in Wisconsin and Michigan, Lake Superior, I sujopose 

 that the iron was originally distributed throughout the soft 

 muds and sands which form the slates of that region ; but by 

 some chemical or chemico-electric agency the iron was gathered 

 together in masses. The denuding agencies which have since 

 swept over the country, have worn away the slates, leaving the 

 iron ore in a mountain shape perhaps two or three hundred feet 

 above the level of the plain. Oxides of iron are found in other 

 formations. "Where the carbonates of iron have cropped out on 

 the sides of hills, they have given off tlieir carbon and taken 

 up oxygen, and have thus been converted into hematites, which 

 are rich ores of this metal. 



Blackband is a carbonate of iron, mingled with bituminous 

 shale. The amount of carbon and bitumen is so great that the 

 whole can be set on fire, and the burning of the carbon will 

 roast the ore. But this ore is only used for mingling with other 

 ores. 



Experience has satisfied iron masters that the best iron is 

 obtained from mixtures of different kinds of iron ores. For 

 instancs, from Pittsburgh they will send to the Adirondack moun- 

 tains for the magnetic oxide, and to Tennessee for the dyestone 

 ore, which they mingle with the nodular ore of their own neigh- 

 borhood. These nodules are found in the coal measures in a 

 shale which is otherwise free from iron. I account for their pre- 

 sence thus : After the coal was formed, there was an irruption 

 into the coal basin of waters from the surrounding neighborhood 

 which carried the neighboring forests and all the floating material 

 into the coal basin. They then became water-logged and sank 

 down upon the coal. Around this vegetable and animal matter, 

 the iron was collected. 



SALTPETRE AND SUBSTITUTES THEREFOR. 



Dr. Stevens. — Saltpetre, or the nitrate of potash, is scarcely 

 ever found as a mineral by itself; but everywhere the elements 

 for its formation exist in the soil, in organic matter, and in the 

 atmosphere. All that is requisite is to protect a sufficient por- 

 tion of the earth from rains and heavy dews. Hence saltpetre 

 or some other nitrate is constantly forming in caves and under 

 barns, wood houses and sheds where organic matter is present, 

 and potassa or some other base is in the soil. During the 



