428 
SILURIA. 
[Chap. XVIII. 
Bigsby, Ramsay, Dana, Billings, and others agree with Hall * and the United 
States' geologists. 
In the lowest of these deposits, at Potsdam, a small Lingula (L. antiqua, 
Hall) was, for a long time, the only fossil known except Fucoids and abundant 
traces of marine Worms (Scolithus linearis). Similar vertical Annelide-burrows 
have been mentioned (p. 40) as occurring in the Stiper Stones of the typical 
Silurian tract, and are equally common in the quartz-rock of Durness t, in 
Sutherland, which underlies the limestone with American types of Mollusca (see 
p. 165). The parallel, therefore, between these British and Transatlantic zones 
seems to be complete. 
Footprints of a large animal were discovered in rocks of a similar age in 
Lower Canada, on the south bank of the St. Lawrence, above Montreal. At 
first they were supposed to have been made by the feet of a Chelonian Reptile ; 
but a further examination of numerous casts taken from the footmarks, and 
brought to England by their discoverer Logan, led Professor Owen to refer them 
to Crustaceans — a class of animals commonly met with in Silurian rocks. This 
view being now adopted, we have evidence of a great protozoic Crustacean, pos- 
sibly not unlike the giant Pterygotus described in an earlier Chapter (p. 162), 
though it may prove to have been more like a Trilobite J. The Lower-Silurian 
fossil, Cruziana, so well known in Britain, France, and Spain, also occurs in this 
lowest zone §. 
The most marked mineral distinction of the rocks which immediately lie upon 
this true Silurian base from those of like age in Britain is the much greater pre- 
valence of limestone. A formation called the 1 Calciferous Sand-rock,' which has 
been paralleled with the crystalline limestone of Sutherland (p. 165), and which 
in Canada is a dolomite ||, is succeeded by the Chazy, Sillery, and other Lime- 
stones, mainly equivalent to our Llandeilo rocks ; and they are followed by the 
great Trenton Limestone with its base of Black -river and Bird's-eye Limestones, 
and its overlying schists (Utica Slate and the Hudson-river Group). In the 
lowest of the great calcareous masses (the Chazy Limestone) the peculiar mol- 
luscous genus Maclurea (see the figures, pp. 165 & 197) is found, together with 
some Corals, Polyzoa, and a few Trilobites (Rlaenus, Asaphus, &c). 
The succeeding calcareous beds (Bird's-eye % and Black-river Limestones) 
forming the base of the Trenton Limestone seem to partly represent the Caradoc 
and Bala rocks of Britain, though, as will be shown in the Table, they enclose a 
larger and more mixed suite of Lower-Silurian fossils. They contain many 
enormous Orthocerata, the singular Cephalopod genus Gonioceras of Hall, large 
Lituites, and several Univalve Shells (Murchisonia, Scalites, &c), besides many 
species of Orthis and Leptaena, including Leptaena sericea, with other Brachiopods. 
The rock into which these beds pass, or the Trenton Limestone, is chiefly, as 
regards its fossils, a fuller development of the last-mentioned deposits, the 
number of Trilobites, Gasteropods, Brachiopods, and Crinoids being vastly in- 
* Lyell took the same view in his ' Travels in 
America,' and also in the early editions of his 
' Manual of Geology ; ' and I cannot but regret 
that in his recent works he has in part abandoned 
it, now that all the explorers of these ancient rocks 
of America have supported my classification. 
t Mention of the occurrence of Scolithus in the 
Lower Quartz-rock of Assynt was inadvertently 
omitted at page 166. The Scottish specimens are 
described and figured in the Quart. Journ. Geol. 
Soc. vol. xv. p. 368, pi. 13. figs. 29, 30. 
I See reduced figures of the Protichnites 
(Owen) in the Eeport Geol. Canada, 1863, p. 104; 
and of Climactichnites (Logan), ib. p. 107. 
$ Principal Dawson has shown that such fossil 
bodies as Cruziana (Eutophycus &c.) may be due 
to the burrowing of some Limuloid Crustaceans. 
Professor Eupert Jones has suggested that both 
Protichnites and Climactichnites have beenformed 
by the great Lower- Silurian Trilobite, — the former 
being tracks, the latter superficial burrows. See 
page 201, note. Professor Dana also alludes to 
these Potsdam tracks as having been caused by 
Trilobites. 
|| See Logan's 'Eeport Geol. Canada,' p. 110. 
•jf So called from the numerous specimens of 
Phytopsis cellulosa which stud the beds and look 
like eyes. 
