LIMITS OF THE VEINS OF LEAD-BEARING STRATA. 
179 
into columns. Its stratification is obscure and its vertical fis¬ 
sures numerous, and which are sometimes prolonged or extended 
horizontally. 
Wisconsin, and the states which are lead-bearing, have been 
subjected both to the denuding action of a rush of waters, and 
to a slower disintegration of rocks by the common atmospheric 
agencies. These external influences have wrought many altera¬ 
tions in the rock, and have also changed the position and 
association of the metal which the rock contains. Originally ’ 
the galena was injected into its fissures, which it undoubtedly 
filled, and which it continues to occupy in part; but in conse¬ 
quence of the disintegrations which the rock has suffered, all 
the upper parts of the veins seem to have been broken down, 
and the lead has become commingled with a stiff, reddish clay. 
The galena is therefore found in veins and in beds. 
LIMITS OF THE VEINS OF THE LEAD-BEARING STRATA. 
§ 107. The lead is not confined wholly to the lower cliff 
limestone. To a limited extent, it reaches the blue limestone 
beneath, though this rock is not by any means so rich and pro¬ 
ductive. The lead, therefore, for all useful purposes, so far as 
is known, is confined to the Niagara limestone, and to the over- 
lying debris which contains the products of the broken down 
veins. 
The fissures vary in thickness from a few lines to fifty feet, 
but Mr. Owen remarks that the common width of the fissures 
filled with solid metal is about four inches, and rarely exceeds 
twelve inches. One instance is given in which the galena of 
a fissure was six or eight feet thick in the center, and extended 
thirty-five feet; it tapered to a point at each extremity. The 
foregoing statement leads to another still more remaikable in 
the annals of lead mining. Thus, fissures sometimes expand 
into large caverns, the walls of which are lined with galena, 
while this is overlaid or incrusted with spar, stalactites, etc. 
The galena which lines these large chambers is sometimes 
