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limonite geodes.^[1] In this case carbon dioxide has been removed from the iron, but oxygen and water have been added. A porosity is also produced by the removal of certain ingredients in an ore deposit without the addition of others, as in the oxidation and leaching of iron pyrites in veins of auriferous quartz, leaving a loose, porous, spongy quartz mass.

Surface decomposition has also, in many places, not only affected the ore deposit itself, but also the country rock in the immediate vicinity, and has converted it into a loose material of a sandy or clayey consistency, as at Iron Mountain, Missouri, in the Batesville manganese region of Arkansas and in other localities described beyond. In the iron and manganese deposits of the Cambrian and Lower Silurian rocks in the Appalachian region, the limestones and shales, which one enclosed the ore bodies, have often been converted to clay in the same way as in the Batesville region; and, in fact, the common mode of occurrence of these deposits is as residual clays carrying irregular bodies and nodules of ore.

This decay of the country rock in immediate association with ore deposits, is generally more extensive than in similar rocks not associated with such deposits, and, therefore, requires further explanation than the simple action of ordinary surface waters. The explanation is, doubtless, in many cases, that the rock has decayed under the influence of the same waters that originally concentrated the ore; and as these waters differed from most waters in character and in the materials they held in solution, they often had an abnormal effect. Moreover, when subsequently the ore body is affected by surface influences, sulphuric acid is liberated from sulphides and carbonic acid from carbonates, as well as other acids from other minerals, and all these materials have an active effect on most rocks. Moreover, the porous nature of many ore deposits, after they have been altered on the surface, allows a freer percolation of surface waters than elsewhere in the same country rock, and, hence, a correspondingly greater decay.