Transactions . original former bark is replaced by a zone of first pyrites, and then (]uartz gminscementeproximatrlypracticable. Fig. 29 is a diagnun of the section of such a stem alten*d togalena. In using altereii into ore as a criterion, solution and deposition and replacement i« particularlyimportant. In the case of larger structures, the distinction is not The genesis of ore de|H3sit«. , Vol. XXIII, p 314) 442 Replacement Ore-Bodies—Irving difficult. For more minute textural features, by one familiar withthe internal structure of calcareous
Transactions . original former bark is replaced by a zone of first pyrites, and then (]uartz gminscementeproximatrlypracticable. Fig. 29 is a diagnun of the section of such a stem alten*d togalena. In using altereii into ore as a criterion, solution and deposition and replacement i« particularlyimportant. In the case of larger structures, the distinction is not The genesis of ore de|H3sit«. , Vol. XXIII, p 314) 442 Replacement Ore-Bodies—Irving difficult. For more minute textural features, by one familiar withthe internal structure of calcareous organisms the characteristicstructure of a fossil may be readily determined by the those who lack such knowledge and in cases where the re-placing mineral is opaque like sulphides, a more satisfactory dis-tinction between cast and replacement is Fig. 29.—Section of a tree stem replaced with galena, Freihung,Bohemia. Shows annular rings of growth preserved in ore. (After Posepny,\. Vol. XXIII, 0pp. p. 366, Fig. 84.) This may often be carried out by the position occupied by theincluded fragments of unreplaced rock. In some animals theoriginal shells, or septse of the original shells, include spaces whichare wholly, or almost wholly enclosed by shell walls. Wheresuch shell walls are changed into a different material, possiblyon account of the superior solubility of the shell material overthat of the surrounding and included rock, it is obvious thatsuch a mass of new mineral could not have formed by solution andredeposition, for the solution of the surrounding septum wouldhave removed the support from the enclosed kernel so that itwould come in contact with the outside rock mass at some point. C^iCiTe
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