Quantitative microscopic analyses from thin sections of the Clinton iron ores of New York and associated rocks reveal a succession of introductions and replacements which were responsible for formation of the ore. The iron was introduced by surface waters shortly after deposition and replaced a series of sedimentary minerals, chiefly carbonates, as part of the diagenesis of the rock. For this, the expression, diagenetic replacement, is here proposed.

Recently developed stratigraphic diagrams, employing the concept of “microlithologies”, indicate that these ores were originally limestones which were replaced by iron compounds, now largely hematite, provided certain essential conditions prevailed and the clay content was less than 10 per cent by weight. The oölitic coatings consist of hematite and chamosite in alternating thin layers, onion fashion. The “fossil” ores are the result of the partial replacement of the carbonate of fossils and the subsequent oölitic coatings. These replaced fossils act as nuclei.

The Clinton ores are lenticular and possess little or no stratigraphic significance; they represent similar lithologic conditions.

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