8 
HAROLD'S DISCUSSIONS. 
rock in the greater part of tlie State, extending in an 
irregular strip up through Arkansas (Fig. 6). Tliese 
rock layers in no place extend beneath other layers. 
St.P. 'L.M. 
Fig. 4. —Cross-section from Lake Michigan west. II.31., Ham¬ 
ilton rock; iV, Niagara limestone; II. R., Hudson River 
(Cincinnati) shale; 6^, Galena limestone; T, Trenton lime¬ 
stone ; St.P., St. Peter sandstone; L.3I., Lower Magnesian 
limestone. 
We therefore conclude that they are the most recent 
chapter of the earth-book. For convenience, and not 
to introduce too many names here, I will call this 
formation dumber 7. 
Fig. 5. —Cross-section from Lake Michigan west, south of Mil¬ 
waukee. P, Potsdam sandstone; L.3I., Lower Magnesian 
limestone; St.P., St. Peter sandstone; G, Galena limestone; 
C, Cincinnati shale; N, Niagara limestone. The forma¬ 
tions above St. Peter sandstone have all been carried away 
by erosion, except at Blue Mounds and a few other high 
places. 
As we go northward we come upon Number 6, a 
slightly different rock formation. It crops out in the 
State of Mississippi and north as far as southern 
