434 
SILTJRIA. 
[Chap. XVIII. 
beds with Corals and Pentameri, all traces of the equivalents of the Ludlow 
rocks being absent. The Devonian period, however, is clearly defined, particu- 
larly by its broad- winged Spirifers ; whilst the Carboniferous Limestone is much 
expanded, and a large portion of it has been distinctly referred to the 1 Yoredale 
series ' of Phillips in Yorkshire. The western explorations of Captain Stansbury 
to the Mormon Territory and the Salt Lake of Utah have further shown us 
that strata charged with Carboniferous fossils extend to the edges of the Rocky 
Mountains, those great northern extensions of the Andes. 
Palceozoic Rocks of British North America. — 'The oldest of these masses (the 
Laurentian) has been described in the beginning of this work. The next in 
ascending order, the Huronian, or the lower copper-slates and sandstones of the 
great lakes, have, to some extent, the same structure as considerable portions of 
the Longmynd or Bottom rocks of the Silurian region *. They are, in truth, 
defined as such by Logan, being termed by him the equivalents of the ' Cambrian/ 
so defined by De la Beche, Ramsay, and the British Government Surveyors, as 
well as by myself. In England and North Wales these old slaty rocks contain 
little or no limestone ; but in North America they are largely interlaced with it. 
Again, whilst in Great Britain and Germany these Cambrian rocks are suc- 
ceeded conformably by the Lower Silurian, they have been severed therefrom 
in America, as in France and Ireland, by a grand dislocation. In thus appeal- 
ing to the conditions in different countries, we learn the impracticability of 
classifying sedimentary deposits according to age by the lines of fracture and dis- 
memberment alone. In making this statement, however, I do not prejudge the 
views of my eminent friend M. ^ lie de Beaumont, concerning the directions of 
mountain-chains as typical of certain epochs. This topic, requiring much illus- 
tration, would lead me away from the special objects of this work. 
In the last few years great advances have been made in unfolding the features 
and in fixing the eras of the formation and dislocation of all the Palaeozoic rocks 
of Canada ; and I am most grateful to Sir W. Logan, the Director of the Geo- 
logical Survey of that great Colony, for having furnished me with the following 
information : — 
a 1. Silurian Rocks. — TheSilurian rocksof North America present in their lower 
portion very marked differences in different areas. This the Geological Surveyors 
of Canada have explained by showing that the Lower Silurian rocks of the New 
York system represent a portion only of the great succession of sediments which in 
the earliest of those periods were deposited in the great ocean surrounding the 
Laurentian and Huronian nucleus of the present American continent. Oscilla- 
tions of this ancient land permitted the spreading out on its surface of those 
sheets of sediment which constitute the Lower Silurian formations of New York 
and Central Canada ; but between the periods when the Potsdam Sandstone 
and the Chazy Limestone were formed, a rapid continental elevation and sub- 
sequent gradual depression allowed a great intermediate accumulation of de- 
posits, which are now displayed in the Green-Mountain range on the one side 
of the ancient nucleus, and in the metalliferous series of Lake Superior on the 
other, while they are necessarily absent in the intermediate New York series. 
u A great dislocation along the eastern line of the ancient gneissic continent 
commenced at a very early date in the Lower Silurian period, and gave rise to 
the division which now forms the eastern and western basins. The western 
basin includes the strata which extended over the surface of the submerged 
continent, together with the Pre-Chazy rocks of Lake Superior; while the 
* For a complete risumd of the relations of the Huronian and Cambrian rocks see Dr. Bigsby's 
memoir in the Quart. Journ. Geol. Soc. vol. xix. p. 36. 
