THE EARTH-BOOK. 
7 
on it. Farther west we find a different kind of lime¬ 
stone known as the Trenton, extending beneath the 
Galena, and hence it must be older than the Galena 
strata. Continuing oiir examination westward across 
a small valley we come upon a layer of St. Peter 
Moraine 
Chaquamegon 
Bag 
Lake 
Superior 
Fig. 3. —The north end of the cross-section mentioned under 
Fig. 2. Strata much disturbed from their horizontal posi¬ 
tions. L, Laurentian (granite group); H, Huronian (gran¬ 
ite group) ; K, Keweenawen (copper-bearing group) ; P, 
Potsdam sandstone. The surface-rock of the cross-section 
between Figs. 2 and 3 is Laurentian granite. 
sandstone. On a ridge west of this is an outcrop of 
Lower Magnesian limestone, then a Potsdam layer, 
and so on. If we trace any of these layers, we find 
that they do not lie horizontally, hut incline and ex¬ 
tend under a layer which must be younger. If a 
well should be bored anywhere in the Galena out¬ 
crop, it would pass through these various strata and 
in the same order in which we found them while 
walking westward. The youngest rock found here, 
then, is the Galena limestone. If, however, we ex¬ 
amine the rock strata elsewhere, we shall find that 
the Galena is not a recent formation, as many differ¬ 
ent strata lie above it in other parts of this and other 
countries. 
Let us now go to the Gulf coast and begin in 
Louisiana. Here is a strata that forms the surface- 
