178 
AMERICAN GEOLOGY. 
limestone is not destitute of lead and zinc; it is in a sufficient 
quantity to keep up the analogy between the beds of rock of 
the same age at distant points. These limestones are both 
magnesian, though the magnesia is variable in quantity in the 
different layers. 
The composition of the lead-bearing rock of Wisconsin was 
found to be as follows: 
Carbonate of Lime, 
47-40 
“ Magnesia, 
40-70 
Oxide of Iron, 
2-40 
Silex, 
7*10 
Water, 
2-00 
Loss, 
40 
100 -00* 
The texture of the rock differs in different places; in some it 
is friable like a sandstone; in others, hard and durable. It is 
also cherty or traversed by bands of compact quartz resembling- 
flint. 
This rock is generally favorable both for the reception and 
retention of the metals. It has been fissured extensively, and 
these fissures had remained open until they received the lead or 
copper or other metals which we find in them. 
A fresh fracture of the rock exhibits a sub-crystalline aspect, 
a flat conchoidal surface, a granular texture, and a light yel¬ 
lowish or drab color. When weathered it is brownish or red¬ 
dish yellow. Specific gravity 2 ’ 6 5 to 2 ‘ 70. 
Owen divides the Cliff limestone into, 1, upper or shell beds, 
consisting of a white or light colored limestone, destitute of 
magnesia, fossils calcareous; 2 , middle or coralline beds, which 
are cherty and magnesian, and of a yellowish color, fossils sili- 
cious; 3, lower or lead-bearing beds, color yellowish and com¬ 
position magnesian. The rock presents cliffs when it appears 
at the outcrop, which are more or less fissured and separated 
*D. D. Owen’s Rep. of Geol. Expl of Iowa, Wisconsin and Illinois, 1836, 
p. 24. 
