1210 



A TREATISE ON METAMORPHISM. 



place. In some places in the district where the capping shale still remains, 

 apparently little or no secondary concentration has taken place; but at 

 many places the capping- shale has been largely removed, and in such areas 

 descending waters have produced the oxidized ores above the level of ground 

 water, have enriched the sulphides below it, and have formed the vertical 

 arrangement of the lead and zinc ores described on pages 1144-1145. 



In the upper Mississippi Valley district it is believed that the lead and 

 zinc have undergone two concentrations, as in the Missouri- Kansas district, 

 although the evidence is not quite conclusive. Here Galena limestone is 

 overlain by the Cincinnati shale, in Iowa called the Maquoketa shale, 



surface 



Fig. 31.— Ideal vertical section of flow of underground water in the Galena limestone of the upper Mississippi Valley. 

 When the surface was at A A' A" the first concentration by ascending water took place; when erosion had 

 reduced the surface to B B' the second concentration by descending water occurred. 



and is underlain by the Trenton limestone, with an oil-bearing shale 

 between. When the impervious overlying shale was partly removed an 

 artesian circulation was inaugurated which resulted in the first concentra- 

 tion. Later, when the overlying shale was largely removed, a second 

 concentration by descending waters took place, which has continued to the 

 present time. (See fig. 31.) 



The main difference between the upper Mississippi Valley and Missouri- 

 Kansas districts is that in the upper Mississippi Valley district the important 

 metals are believed to have their chief source in the limestones in which 



