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disregarding all theories and all possible explanations of any- 
thing which may appear unintelligible. For reasons already 
stated; I shall select for this inquiry the Upper Silurian and 
Devonian rocks of North America as being peculiarly fitted for 
this purpose. AYe have here a series of distinct rock-groups, 
all of which are capable of being defined by their fossils, but 
which follow one another conformably, and which .possess a 
sufficiency of identical or closely-allied fossils, in any two 
successive groups, to indicate that they constitute a single 
natural group of deposits, elsewhere represented by the Upper 
Silurian and Devonian. When most fully developed, the series 
consists of the following groups in ascending order : — 
I. Upper Silurian. 
1. Oneida Conglomerate. 
3. Clinton group. 
4. Niagara group. 
5. Guelph Limestones and Onondaga Salt group. 
G. Lower Helderberg. 
a. Tentaculite Limestone and Water-lime group. 
b. Lower Pentamerus Limestone. 
c. Delthyris Shaly Limestone. 
d. Encrinal Limestone. 
e. Upper Pentamerus Limestone. 
II. Devonian. 
7. Oriskany Sandstone. 
8. Corniferous group. 
a. Cauda-galli grit. 
b. Schoharie grit. 
c. Upper Helderberg or Corniferous Limestone. 
9. Hamilton group. 
a. Marcellus Shale. 
b. Hamilton group proper. 
c. Genesee Slates. 
10. Portage group. 
11. Chemung group. 
12. Catskill group (Carboniferous?) 
The line of division between the Upper Silurian and Devonian 
is so little marked that the best authorities are still divided as 
to w hether the Oriskany Sandstone should properly be regarded 
as the summit of the former or the base of the latter ; and it 
may conveniently be regarded as constituting a bed of passage 
between the two. In what follows, several groups of the above 
list will not come into consideration at all, as not yielding 
2. Medina Sandstone. 
