48 IOWA ACADEMY OF SCIENCES. 



required in the mappiug of any other formation in the Archaean regions. 

 Where canons occur in the granite their walls are more rugged, less pre- 

 cipitous and higher than in the porphyry, and the waterways are broader. 



Closely associated as they are, the crystallines and the Cambrian ledges 

 exhibit the contact of the two formations in many places. In some por- 

 tions of the region it is not uncommon to find sandstone more closely 

 accompanying the granites; limestone, the porphyries. Dikes are found 

 more commonly in the granite. Iron ores are found mainly in the por- 

 phyry, while the lead ore in the crystallines is confined largely to the 

 granite. 



Specimens from many localities show an almost continual change in the 

 hue, if not in the texture, both of the granites and porphyries. Between 

 fifty and one hundred hues ai'e represented in the former, while in the latter 

 about two hundred distinctly different hues are shown, each in the cor- 

 responding number of specimens collected. 



Associated as a dike rock in the granite, olivine diabase is also found, 

 making up a few areas of considerable size. These have a topography much 

 like that of the smaller granite fields. Limited areas of so-called syenite 

 occur; also other forms of crystallines, rocks which will not here be men- 

 tioned. 



OCCURRENCE OF ZINC IN NORTHEASTERN IOWA. 



BY A. G. LEONARD. 



In the Upper Mississippi valley for a considerable period after the mines 

 began to be operated much more lead than zinc was produced. It was not 

 until 1860 that the latter metal came into market. Since then the zinc pro- 

 duction has rapidly increased. During the ten years previous to 1882 the 

 output of zinc more than doubled that of lead, while in 1889, according to 

 the last federal census report the proportion between the two was as 13 to 1 

 for the entire region. 



On account of their increasing importance the zinc deposits will be 

 especially described in this paper, but as the two metals are so closely 

 related in occurrence what is said of one will, in many cases, apply equally 

 well to the other. 



Not until the year 1880 were the Iowa mines worked for zinc carbonate 

 or "dry bone," as it is called by the miners. Up to that time the carbonate, 

 though found in many of the mines, was thought to have no special value 

 and had been throvvn away as worthless, or when found in the diggings the 

 latter were abandoned. 



In the fall of 1880 two wagon loads of zinc were taken to Benton, Wis- 

 consin, by Mr. William Hird and sold for $16.00 per ton. So far as known 

 this was the first zinc ore sold from the mines of the State, and from this 

 time on the carbonate has been mined in rapidly increasing amounts. The 



