HAROLD’S DISCUSSIONS. 
The rock formations of one chapter—that is, those 
of one age—are distinguished from those formed in 
another age, not so much bj the character of the 
rocks as bj the fossils of the life that existed during 
that period. 
A comparison will show that the fossils of certain 
belts are much alike, and quite different from those in 
an adjacent belt. The conclusion that these unlike 
groups of animals lived in two unlike periods of time 
is evident, and the strata that lie lowest are, of course, 
the oldest. 
Great periods of time are distinguished bj great 
differences in their fossils, and short periods bj cor¬ 
respondingly small differences. The division is some¬ 
times based on only one or two forms whose remains 
make up a certain percentage of all the fossils found. 
Let us examine the rocks exposed to view near 
Marquette, Wis. At the lake is a slight deposit 
of sand which the water has left. Going west and 
a little north, we come upon a kind of limestone 
which the expert at once recognizes as Galena lime- 
L.M. 
Fig. 2.— The south end of a cross-section of surface-rock of 
the State of Wisconsin, from Ashland south by east to 
Lake Michigan. 
stone. This layer, as may be seen from Fig. 2, ex¬ 
tends beneath the recent sand deposits, and of course 
it had been formed before the sand could be deposited 
