34 WILLIAM H. EMMONS 



many ways by which acidity is reduced along with these reactions, 

 but the principal one is probably the kaolinization of sericite 

 and feldspar. In these reactions sodium, potassium, calcium, 

 magnesium, and other sulphates are formed from acid and silicates; 

 the silica remaining as Si0 2 and kaolin; the alkalies and alkalic 

 earth sulphates going into solution. As the acidity decreases, 

 iron and manganese compounds tend to hydrolyze and deposit 

 oxides. At this stage of oxidation FeS0 4 becomes increasingly 

 prominent, and not only completely inhibits further solution of 

 gold but becomes increasingly effective as a precipitant. Thus 

 manganite is probably precipitated with gold. The fractures in 

 the primary pyritic gold ore below the water level thus become 

 coated with a manganiferous gold ore, which may be very rich. 

 The excess of oxygen which the system has carried down is used 

 up in the manner indicated, and in this process limonite is formed, 

 consequently the manganiferous gold ore deposited in the fissures 

 and cracks contains kaolin and iron as well as manganese oxides. 

 2. The oscillating, descending, undulatory water-table.— -The 

 terms "water-table" and "level of ground- water " are generally 

 used to describe the upper limit of the zone in which the openings 

 in rocks are filled with water. This upper limit of the zone of 

 saturation is not a plane, but a warped surface. It follows in 

 general the topography of the country, but is less accentuated. 

 It is not so deep below a valley as below a hill, but it rises with the 

 country toward the hilltop and in general is higher there than in 

 the valley. Nor is it stationary. In dry years it is deeper than 

 in wet years, and in dry seasons it is deeper than in wet seasons. 

 The difference of elevation between the top of this zone in a wet 

 year and in a dry year is normally greater under the hilltop than 

 on the slopes and in the valleys. In mines where the ground is 

 open the level of ground-water probably changes with every con- 

 siderable rain. Consequently, there is a zone above ground-water 

 in dry periods but below it in wet periods, and in hilly countries 

 this may be of considerable vertical extent. Thus the water-table 

 oscillates, though in general moving downward with degradation 

 of the land surface. It is in this zone of oscillation of the water- 

 table that chemical activity is most varied. Without any change 



