Chap. X.]                         UPPEE SILUEIAN FOSSILS.                                 215
 

CHAPTER X.
FOSSILS OF THE UPPEE SILUEIAN EOCKS.

The reader has already been told that many species of fossUs, once sup¬
posed to be pecuhar respectively to the Lower or Upper SUurian rocks, are
now ascertained to be common to both, and necessarily to the intermediate
and connecting group.

This datum is the result of the researches of various geologists and
palaeontologists, whether in the region first explored or in tracts of far
greater extent which have recently been paraUeled with it. SimUar results
have, in truth, invariably foUowed from a fuU and broad development of
the natural geological groups of the Secondary and Tertiary strata, which
were described and classified before the older rocks of which we now treat
had been brought iuto order, or even into notice. Thus, for example,
when the difi'erent members of the Oohtic formations reposing on the Lias
were studied in one tract only, as on the eastern coast of Yorkshire, they
were seen to be there composed of a series of zones, each of which is
sharply separated from the contiguous deposits by fossils confined to it.
On tracing, however, the same strata to remote distances, certain remains,
which were once viewed as typical of one member only, were found to be
common to several subformations, thus combining the whole in one natural
system—the Oohtic or Jurassic.

This is just what has happened in the Lower Palaeozoic rocks, now that
the inferior and superior masses, and the chief formations and subdivisions
of SUuria have been ascertained by my cotemporaries to occupy nearly aU
North and South Wales, large tracts of Cumberland, Westmoreland, and
Lancashire, great regions in Scotland and Ireland, and various parts of
Europe, America, and Australia.

In short, the two chief divisions, which, from a general simUarity, were
originaUy grouped together, have been demonstrated to constitute a natural
system, through a community of organic remains. For, even if the con¬
tents of the intermediate middle zone of Llandovery rocks be abstracted
from the estimate, stiU there are many species (from fifty to sixty) which
range from the Llandeilo and Caradoc into the Wenlock and Ludlow rocks.

The vertical range of aU these fossUs through the chief SUurian deposits
is given at the end of the volume, in a Table prepared by Mr. Salter, with
the aid of Professor Morris, and subsequently revised and much augmented
by Mr. Etheridge.

To some of the most striking of these remains which pervade the whole