178 GEKEKAL GEOLOGY. 



sulphuret, /has been found in small quantities in this forma- 

 tion at various localities, but it has never been profitably 

 mined in it outside of Dubuque county. It has also been 

 found in the Trenton and Lower magnesian limestone in 

 other counties, but always, so far as Iowa is concerned, in 

 quantities too small to be profitably mined. 



The lead region proper is confined to the immediate vicinity 

 of Dubuque, and within an area of about fifteen square miles. 

 The ore occurs principally in vertical fissures which traverse 

 the rock at irregular intervals from east to west; a little being 

 found in those which have a north and south direction, and 

 some occasionally in " quartering " lodes or fissures. The ore 

 is mostly that known as Galena or sulphuret of lead; very 

 small quantities of the carbonate only being found with it, 

 and that probably occurring by decomposition of the 

 sulphuret. There is considerable physical difference between 

 the ore of the east and west and the north and south lodes or 

 crevices. Cubical crystals of it are said never to occur in the 

 latter while that is the prevailing form in the others. This 

 probably occurs from the fact that the north and south 

 crevices being very narrow, are usually filled with ore when it 

 occurs at all, and the natural form of the crystals thus 

 confused. But yet there is also a slight difference in color 

 and lustre. 



Prof. Emery has made a full series of analyses for the 

 purpose of ascertaining whether silver is present in any 

 of the ores. The results will be found in his report in 

 volume two. 



The ore has not a uniform vertical range through the form- 

 ation in the Dubuque lead region, but is confined principally 

 to its central portions. Its vertical range may be understood 

 with considerable exactness by the accompanying diagram, 

 (Fig. 5,) which represents the full thickness of the formation at 

 Dubuque. 



