APPENDIX—GEOLOGICAL SURVEY. 
411 
paired we must examine the lower and undisturbed beds of the 
strata. 
To do this, let us take our stand at a point on the Mississip¬ 
pi where we have a good exposure of the lower rocks; we 
will commence at a point just west of Potosi, where we find 
the blue limestone down even with the water level. If from this 
point we draw a line east, or parallel with the mineral belts 
before referred to, we shall find the lower beds of this forma¬ 
tion gradually rising as we approach the center of the district. 
A little east of Potosi, for instance, on a branch of the Platte 
river, we find the sandstone rising from the bed of the stream 
and forming a ledge of rock along its banks, bringing up the 
blue limestone at least fifty feet above its level at our starting 
point on the Mississippi. In following this line across the 
Platte we find the lower beds of the formation still rising. 
Not only do we find the sandstone, but the lower magnesian 
limestone that underlies it, forming ledges fromffifty to seven¬ 
ty-five feet high, bringing up the sandstone and blue limestone 
not less than two hundred feet above its level on the Mis¬ 
sissippi. Further east on this line the lower beds of rock are 
not sufficiently exposed to enable us to determine just where 
the summit of this anticlinal is reached, or just where it com¬ 
mences to dip on the other side. 
If, however, we take our stand on the Mississippi, a little 
further north, about where the Wisconsin river enters it, and 
follow along the line of the Milwaukee and St. Paul Railroad, 
we shall find a section across this elevation that will bring out 
to a great extent its outlines. 
At this point the lower magnesian limestone extends down 
to the water level, or below it, the Potsdam sandstone forming 
the bed of the river. In extending our examinations east from 
this point, we find in a very short distance the sandstone emerg¬ 
ing from beneath the valleys, and gradually rising until it 
reaches a point near the fourth principal meridian. Here we 
find an elevation of the Potsdam sandstone from 200 to 250 
feet above the vallev of the Wisconsin river, and not much less 
than 300 feet above its level at the Mississippi. After extend¬ 
ing along for several miles at about the same level, a very 
