IOWA ACADEMY OF SCIENCES. 51 



esting specimens were seen in which fossils had been replaced by the car- 

 bonate. One of these was a slab of dry-bone on which were several large 

 gasteropods, their substance changed over into the zinc ore, which had pre- 

 served their outline perfectly. The carbonate will contain on an average 35 

 to 40 per cent of zinc. 



The sulphide or "black-jack" of the miners is not found so abundantly 

 in Iowa mines as is the Smithsouite. This is doubtless due to the fact that 

 the former has been largely altered into the latter as will be explained later. 

 The ore contains considerable iron and is so dark colored as to resemble the 

 galena on a cleavage face. 



The silicate is rarely found. When occurring it forms coatings on the 

 the Smithsonite. Some specimens collected had a banded structure and 

 were not unlike quartz in appearance, 



All the carbonate has without doubt been derived from the blende. 

 Several facts indicate this to be the case: (1) It is not uncommon to find 

 pieces, the outside of which ai'e dry bone while the unaltered interior is 

 composed of the sulphide. (2) In the lower levels and where it is below the 

 water the ore occurs as the blende. This is the universal rule and would 

 seem to be owing to the fact that the deposits beneath are not subjected to 

 the alteration agencies at work nearer the surface. 



The zinc ore may occur pure or mixed with more or less clay and rock. 

 The carbonate is found coating the sides and top of the opening and cover- 

 ing the rock fragments in these. As before stated large masses of nearly 

 pure dry bone occur filling large caves. In one of these great cavities the 

 ore was so loosely deposited that a blow of the pick would cause tons of it 

 to come tumbling down. 



la their vertical distribution the lead and zinc ores of Iowa are unlike 

 the occurrences in other parts of the region. Chamberlain makes the fol- 

 lowing statement concerning this: 



" It is a law to which no noteworthy exceptions have yet been authenti- 

 cally reported, that lead predominates in the upper beds, but relatively 

 decreases in the lower, while the zinc ores are very scant in the upper hori- 

 zons, but relatively increase and often predominate below." This law does 

 not hold good for the Dubuque region. There the zinc ore commonly 

 occurs on the same level as the lead, and in some cases even above it. The 

 zinc ore occupies the upper beds of the Galena limestone, few shafts reach- 

 ing a greater depth than 120 feet, and then the upper portion of many is in 

 the Maquoketa shales. It is doubtless true that the majority of the mines 

 are in the upper one hundred feet of the Galena limestone, while in Wis- 

 consin the zinc is confined mostly to the underlying Trenton. It often 

 happens that the lead gives out in the crevices and, a short distance beyond, 

 in the same opening, zinc ore will appear. Why the Galena should sud- 

 denly cease and the carbonate come in within a few yards, is a fact hax'd to 

 explain. The two oi'es I'arely occur mixed together, and where they are 

 mingled the lead is in small quantities. 



It will not be in place hei*e to discuss at any length the theoretical ques- 

 tions connected with the zinc deposits. The subject is a difficult one, and 

 sufficient data are wanting to prove, in a satisfactory manner, some of the 

 theories advanced. But the questions connected with the origin of the zinc 

 deposits are of much interest, both practically and scientifically and will 



