36 



ECONOMIC GEOLOGY 



would represent a metasomatic replacement. The replacing 

 material is introduced by circulating meteoric waters. The sol- 

 vent action of the water is increased by the presence of carbon 



FIG. 24. Contact deposits in limestone beneath shale. 



dioxide under pressure, sulphides and silicates, together with the 

 humic acid of the soil. 



Where sedimentaries lie in contact with igneous rocks, iron 

 is leached out of the intrusive and by chemical action deposited 



FIG. 25. Iron ores of Michigan interbedded with sedimentary rocks. 



(After Emmons.) 



as an iron ore wherever an equivalent amount of limestone has 

 been dissolved and transported elsewhere by percolating waters 

 (Fig. 24). Metasomatic metalliferous minerals are most com- 

 mon in the sedimentary rocks. They may be deposited soon 



