THE ORIGIN AND DESCENT OF ROCKS. 481 



takes place and the action becomes complex. With the waters taking 

 up and throwing down material at the same time, it is difficult to esti- 

 mate the balance of results. 



T\Tien waters that have been mineralized near the surface descend, 

 they often take on a precipitating phase at no great depth below the 

 upper level of the ground-water; thus sulphides that were oxidized 

 and dissolved near the surface are reprecipitated, often at horizons not 

 greatly below the permanent water-level. Waters that dissolve metallic 

 substances in the upper levels often become charged with sulphuretted 

 hydrogen and other precipitants within a few scores or a few hundreds 

 of feet of the surface, as deep wells abundantly prove. The freshness 

 of surface which metallic sulphides often exhibit at these levels is fair 

 ground for inferring recency of deposition and absence of solvent action. 

 Actual demonstrations of depositions in progress are not wanting. 



Short-course action. — The concentration which thus takes place by 

 solution in the upper zone, followed closely by reprecipitation ^dthin a 

 few score or a few hundred feet, may well be termed the short-course 

 mode of ore concentration. It finds its most important illustration in 

 what is commonly known as the ^'secondary enrichment " of ore-deposits. 

 The ores in the outcropping edge of the vein or lode are dissolved by the 

 surface-waters, carried a short distance down the ore tract and rede- 

 posited, causing enrichment at that point. This is only a special case 

 of what takes place generally at this horizon. It is effective in this case 

 because it has a previous partial concentration to work upon. Secondary 

 enrichments of this kind often contain most or all the workable values 

 of the ore tract. If instead of a previous concentration in a vein, lode, 

 or similar ore tract, there had been partial concentration in the country 

 rock by sedimentation, as in the case of iron-ore beds and perhaps lead-, 

 zinc-, and copper-impregnated sediments, the short-course method may 

 give working values not before possessed. In some of the more obscure 

 cases of previous partial concentration in the crystalhne and other rocks, 

 it is probably this short-course action that brings the concentration 

 up to working value. It is probably effective also in concentrating 

 the metalHc contents of certain igneous rocks that were rich in metallic 

 material when extruded. How far this is true has been, and still 

 remains, a mooted question. 



Long-course action. — After the surface-waters have once passed 

 through a cycle of dissolving and precipitating action, as they are apt to 



