LEAD AND ZINC ORES. 1151 



the oil rock, which have by denudation passed into the belt of weathering, 

 numerous partially dissolved crystals of galena in openings and many 

 cubical openings once evidently occupied by galena but now wholly vacant 

 show. abundant evidence of oxidation and solution of lead sulphide. 



Whether or not the reactions written above express the exact chemical 

 changes, it is certain that oxidized lead salts are precipitated by the sul- 

 phide of iron and by the sulphide of zinc as lead sulphide, as was shown 

 by the experimental work of Anthon and Schiirman already referred to. 

 (See pp. 1114-1115.) 



So far as my argument is concerned, it is of no consequence whether 

 the lead be transported as a sulphate, carbonate, chloride, or other salt. 

 However, it is believed that these are the forms in which the lead was trans- 

 ferred on the most extensive scale. I regard the cerussite and anglesite as 

 evidence of the partial transfer of the lead as sulphate and carbonate. A 

 large amount of siilphate and carbonate probably formed, but the com- 

 pounds are so insoluble that a part of the salts produced was not carried 

 downward, but precipitated near the places of formation. 



In the upper Mississippi Valley for a short distance above and below 

 the level of ground water there are cubic crystals of galena of very large size 

 as compared with those disseminated through the sphalerite at lower levels. 

 The crystals at this upper horizon are commonly from 5 to 8 centimeters 

 in diameter, and a coiisiderable proportion have diameters of 10 centimeters 

 and some of 15 to 20 centimeters. In contrast with this, the galena 

 intimately associated with the sphalerite at the lower levels is very rarely 

 in crystals larger than 5 centimeters in diameter, while a large j:>art of it 

 is in smaller particles. It is thought probable that the large size of the 

 crystals at the upper horizons is the result of additions made by descending 

 water to the smaller crystals of the first concentration Below the level of 

 ground water octahedral crystals of galena are frequently superimposed 

 upon the cubic crystals of this compound. 



sphalerite. — Evidence of the oxidation of sphalerite and the transporta- 

 tion of the material elsewhere is as clear as in the case of galena, both in 

 the upper and lower Mississippi Valley. At many places in southwestern 

 Missouri sphalerite crystals may be seen in the chert in various stages of 

 oxidation, and the porous rocks contain numerous casts of the sphalerite 

 crystals, now vacant or occupied by a film of iron oxide. Precisely the 



