312 Professor John W. Greg or ij] [April 27, 



measures, the inarlstones of the Lias, and the ores which in Roman 

 and mediifival times, maintained the iron industry of the Weald. 



Such ores are deposited contemporaneously with the rocks in 

 which they occur ; but other iron ores have been formed by the 

 infiltration of iron salts into pre-existing rocks. Thus have been 

 formed the masses of valuable kidney iron ore in the limestones of 

 the North of England. They have been formed where a bed of 

 limestone is covered by a sheet of sandstone stained red by iron oxide. 

 Rain water, charged with carbonic acid from the air, or with organic 

 acids derived from the decay of plants, percolates through the red 

 sandstones, removes the iron as a soluble carbonate, and carries it 

 down into the underlying limestone. Here, the soluble iron salt is 

 acted on by the limestone, some of which is removed in solution, and 

 iron oxide precipitated in its place ; and thus, the masses of kidney 

 ore are slowly formed, and grow at the expense of the surrounding 

 limestone, as descending waters nourish them with fresh supplies of 

 iron. 



The distribution of such ores is limited by two factors. They 

 are limited in area to localities where there was once an overlying, 

 permeable, iron-stained bed to supply the iron ; and they are limited 

 in depth by the thickness of the limestone, and that to which water 

 can descend through that rock, before the precipitation of all its iron. 



The great deposits of hcfimatite in the Lake Superior region, 

 which have given the United States its supremacy in iron production, 

 were, according to Van Hise, similarly formed by descending oxy- 

 genated solutions, caiTying down iron from beds of iron carbonate. 

 As the oxygenated waters necessary for this process cannot descend 

 to any great depth, it is probable that the main mass of these ores will 

 be limited to the depth of 1000 feet. 



The Ijeds of clay ironstone formed as contemporary deposits, may 

 on the other hand go down to great depths, when sunk in folds 

 beneath thick beds of overlying sediments. The depth of such ores 

 is a simple prol)lem in stratigraphical geology, and the limit to which 

 they can be worked is determined only by the cost of mining. 



These iron ores derive their materials from igneous rocks indi- 

 rectly ; but there are some iron ores of direct igneous origin. They 

 have been formed by the segregation of the metallic constituents of 

 a molten magma into masses, or into bands along the cooling edges. 

 The existence of such primary ores is indisputable ; but their amount 

 is still largely in doubt. They include masses of magnetite, and also 

 ores of chromite, and according to some authorities, masses of copper 

 pyrites, and of nickel-bearing pyrhotite, and even cassiterite. There 

 is no reason why these segregations should not form at any depth at 

 which molten rocks consolidate ; for both the pressure and heat 

 would facilitate their formation, by allowing the molten work to cool 

 so slowly that the segregation can collect the metallic constituents 

 from a vast bulk of rock. Such igneous segregations usually occur 



