268° Cincinnati Society of Natural History. 
lead as to the mineral or petrological structure or relative position of 
the strata, and it indicates the typical locality of the exposure. 
Sandstones and conglomerates of various degrees of fineness, lime- 
stones of all grades and combinations, shales and marls occur in nearly 
every group in the Lower Silurian, Upper Silurian, Devonian and Car- 
boniferous : and, for this reason, geological subdivisions can not be 
established upon the mineral and petrological characters of the rocks. 
This has been demonstrated too frequently to demand further consider- 
ation. 
The subdivision into groups is founded, substantially, if not entirely, 
upon the paleeontological characters. The petrography may suggest 
the subdivision and creation of the group, but its establishment de- 
pends upon the fossil contents. The fact, however, that the fossils, 
from contemporaneous animals, must change more or less, in different 
degrees of latitude, and in deposits made at various depths of the 
ocean, and be more or less dependent upon the ocean currents that 
prevailed, and upon the nature of the sea-bottom, which formed their 
habitat, has rendered it no easy task to determine equivalency of strata, 
at short distances, and the difficulties and troubles increase as the pe- 
trographic characters change and distance intervenes. The perplexi- 
ties thus arising are usually overcome, in proportion to the thickness 
of the strata, which are understood to constitute the group, the greater 
or less number of species that have been described from it, the more or 
less accurate information respecting the grouping of the fossils in its 
various parts, and the different kinds of rocks in which they are thus 
grouped or associated. It happens, sometimes, that a group is so de- 
fined, at one locality, as to show that it is different from any previously 
defined, and is therefore seemingly worthy to be established ; but 
afterward, from more complete study and comparison of the fossils, 
it is ascertained that such group is included, in some manner, 
within a larger one defined elsewhere, and constitutes part of it, and 
yet the equivalency of the strata can not be ascertained. In such case, 
we have synonymy and still both names are usually retained. Again, 
when groups are separable, and have been separately defined, at one 
locality as the Medina, Clinton, and Niagara Groups of New York, we 
find that some authors, describing the series of strata equivalent to 
these groups, at another place, where they can not be readily dis- 
tinguished from each other, will propose for them when combined a new 
name, as the Anticosti Group. 
Or, perhaps, a better illustration of synonymy of the latter kind is 
: 
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