THE KING OF METALS. 129 



the iron was transported and accumulated in spheroidal or 

 concretionary forms in a state of solution hi water. It must, 

 therefore, have existed as a protoxide, and must have com- 

 bined with further oxygen or with carbonic acid subsequently. 

 When combined with the latter, it forms iron carbonate, and 

 this is one of the ores of iron. As an ore, it is siderite. It 

 possesses various degrees of purity. Often it occurs as a con- 

 cretion five to eight inches long, formed in the rock as I have 

 just explained. It may thus embrace much sand or clayey 

 matter, and this is the condition in which the siderite nodules 

 or " clay iron stones" are found in the coal measures and 

 other formations. 



So you perceive that iron ores do not occur in proper 

 veins. They are isolated masses, or they are strata. They are 

 not mined out through shafts and drifts and chambers, like 

 the ores of gold and silver, but mostly in open excavations. 

 In Salisbury, Connecticut, the excavations extend into cav- 

 ernous, deranged stratified rocks, and many of the cavities are 

 lined with a black, polished coating of ore which when 

 scratched is yellow, and therefore limonite. This limonite has 

 been in solution. It has filtered through the interstices of 

 the formation. In many of the cavities are beautiful stalac- 

 titic forms hanging from the roof, or stretching from roof to 

 floor. These are much sought as fine specimens for the 

 cabinet. 



The mean specific gravity of the whole earth is twice that 

 of the heaviest rocks. Is that due to compression of the in- 

 terior, or to the presence of some substance heavier than the 

 ordinary materials at the surface? Some have suggested the 

 probability that the earth's central mass is a vast ocean of 

 molten iron. It will be remembered, also, that iron is a chief 

 constituent in meteoric masses. 



