SUPERFICIAL ALTERATION OF ORE DEPOSITS. 295 



limonite geodes.' In this case carbon dioxide has been removed 

 from the iron, but oxygen and water have been added. A poros- 

 ity 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 local- 

 ities described beyond. In the iron and manganese deposits of 

 the Cambrian and Lower Silurian rocks in the Appalachian 

 region, the limestones and shales, which once 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 occur- 

 rence 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. 



^ R. A. Y. Penrose, Jr., The Tertiary Iron Ores of Arkansas and Texas, Bulletin 

 Geological Society of America, Vol. 3, 189 1, pp. 44-50- 



