430 
SILUKIA. 
[Chap. XVIII. 
the simple terminology of Lower and Upper Silurian, — grouping the Lower 
Llandovery rocks with the former, and the Upper Llandovery with the latter 
division (see Chapter V.). 
This method is, it seems to me, preferable, because this central group, which 
unites, byits fossils, the inferior and superior masses, is often not rich in organic 
remains, and in some countries (as in Bohemia) cannot be easily separated from 
the subjacent and overlying deposits. 
The Clinton Group is immediately followed by the Niagara Shale, which in 
all respects resembles the well-known Wenlock Shale of Britain ; whilst the chief 
or central mass of the Upper Silurian rocks in North America is that called the 
Niagara Limestone, which unquestionably represents the Wenlock and Dudley 
Limestones of England, as well as of Gothland in the Baltic. These rocks 
appear to contain a greater number of fossils identical with those of Europe 
than do the Lower Silurian strata of the same districts. Among them are 
Calymene Blumenbachii, Homalonotus delphinocephalus, Bumastus Barriensis, 
Rhynchonella cuneata, Rh. Wilsoni, Pentamerus galeatus, Orthis elegantula, 
0. hybrida, Orthoceras annulatum, Eucalyptocrinus decorus, and Bellerophon 
dilatatus. (See Chaps. IX. and X.) Some of the large European Corals also 
prevail — those which may have been capable of forming reefs, as Favosites 
Gotlandicus, F. alveolaris, and the Chain-coral. There are also many peculiar 
species : indeed Professor Milne-Edwards has shown that out of a considerable 
list of American Corals few are really identical with those of Europe. 
The lower formations of the next overlying or Helderberg division, up to the 
higher Pentamerus-limestone inclusive *, constitute (according to de Verneuil) 
the probable equivalents of the Ludlow rocks. The much greater abundance of 
calcareous matter in the higher part of this division than exists in our own 
Ludlow rocks, and the absence of that muddy and sandy matrix which charac- 
terizes those strata in Britain, have necessarily given to the whole group more 
the character of the Wenlock formation, — a feature which, as we have already 
seen, is dominant in Gothland, Russia, Bohemia, &c. 
2. Devonian Mocks. — The base of the Devonian rocks of the United States has 
been placed lower than it was in the earlier comparisons with European de- 
posits. Seeing that a mass of sandstones and conglomerates had there been 
called 1 Old Red Sandstone,' it was natural, in the first instance, for geologists 
to suppose that the Devonian series in North America did not descend far 
beneath this rock. Hall and others, however, have long since agreed with de 
Verneuil in considering the Oriskany Sandstone to be the true Devonian base ; 
for this rock contains Spirifer macropterus, Pterinea fasciculata, and Pleurodic- 
tyum problem aticum, all species of the lowest Devonian of the Rhine, Belgium, 
and France, which are unknown in any Silurian rock (see Table facing p. 405). 
The lowest Devonian rock in America is therefore precisely similar to its type 
in Western Europe. 
A further reason for placing the Oriskany Sandstone at the base of the De- 
vonian rocks of New York consists in the clear indications of its having been 
deposited in excavations in the inferior stratum, as explained by Mr. J. Hall. 
This phenomenon marks a previous denudation, and the commencement of a new 
series f . 
* These consist, in ascending order, of the Onon- American deposits, whether in the State of New 
daga-salt Group, the Water-lime Group, Lower York or in the adjacent British territories, has 
Pentamerus-limestone, Delthyris Shaly Lime- been clearly illustrated by Dr. Bigsby, one of the 
stone, Encrinital Limestone, and Upper Penta- most able and energetic explorers of those regions 
merus-limestone. in former years. The general succession is that 
t The subject of the succession of the North- given in the Table at the end of this Chapter. Di- 
