288 The America)! (ieoloyist. November, i895 
ORIGIN OF THE IOWA LEAD AND ZINC 
DEPOSITS.* 
By A. (i. Leonard, Toledo, Iowa. 
The lowH lead and zinc deposits occur in the nortlieastern 
corner of the state and form part of a larger area known as the 
Upper Mississippi lead and zinc region to distinguish it from 
the Lower Mississippi or Missouri region. The Upper Missis- 
sippi area has a length east and west of 96 miles and a width 
north and south of 55 miles, embracing the southwest portion 
of Wisconsin, northwest corner of Illinois and adjoining parts 
of Iowa. This region, embracing some 3,000 square miles, lies 
wholly within the limits of the driftless area. 
As is well known, the lead and zinc occur in crevices in the 
Galena and Trenton limestones. These formations are cleft 
by extensive east and west fissures, which at certain depths 
are found to expand into cave-like "openings;" it is in these 
openings that a large portion of the ore occurs. In Iowa the 
first and largest "opening" is as a rule about 45 feet below the 
top of the Galena beds and most of the ore has been taken 
from the upper fifty or sixty feet of the limestone. 
The question of the origin of the lead and zinc deposits has 
been under discussion ever since the days of Owen and Perci- 
val and has recently come into renewed prominence through 
the work of Winslow, Jennej'-, Blake and others. New light 
has been thrown upon the subject, new tjieories advanced and 
old ones reaffirmed. 
In treating of the genesis of these deposits it will be well to 
consider in the first place the original source of the lead and 
zinc and the way in which the ores came to be confined to cer- 
tain districts, and then to discuss the formation of the crevi- 
ces and the deposition of the ores in these receptacles. 
Original source of the lead aud zinc. For the source 
whence the minerals were originally derived we must doubt- 
less look to the primitive Archean rocks forming the land mass 
to the north. As this land was wasted away and its materials 
carried into the Silurian sea, the waters became charged with 
metallic salts which were deposited along with the limestone. 
The chief agent in the precipitation of the metals appears to 
have been the organic life so abundant during this period. 
The death and decay of the vast multitudes of mollusks and 
*Published by permission of the State Geologist of Iowa. 
