424 
SILURIA. 
[Chap. XVIII. 
CHAPTER XVIII. 
SUCCESSION OF PRIMEVAL ROCKS IN AMERICA. 
ORDER OF THE PALAEOZOIC ROCKS IN SOUTH AMERICA (THE ANDES), THE UNITED STATES, 
AND BRITISH NORTH AMERICA. 
The researches of geologists have demonstrated that there was a wide 
diffusion of similar groups of animals over the globe during the Primeval 
Periods. A striking proof of this fact is, that many of the Palaeozoic fossils 
which we have followed over the various countries of Europe are found to 
have unquestionable equivalents in the continent of America. 
It is in America that the discovery of animal life in the Laurentian 
or oldest known sedimentary rocks was made, which has been followed by 
a similar discovery in Europe. The highly important researches of Logan 
and his associates, demonstrating that all the Palaeozoic rocks, so well de- 
scribed in the United States by Hall and others, repose upon the double 
series of highly metamorphosed gneissic strata in which the Eozoon Ca- 
nadense occurs, have been explained in the First Chapter of this work (p. 11 
et seq.). These Laurentian rocks, though very extensive in Forth America, 
constitute a low mountain- chain compared with the much younger and 
loftier Andes, which, as described by Humboldt, form the main geogra- 
phical axis of the great western continent. 
The oldest of the slaty and quartzose formations so admirably delineated 
by that great traveller, in whose youthful days fossils were little studied, 
have since been referred, by means of their organic remains, to the Silurian 
System. Following up the inquiries of his precursor, Alcide d'Orbigny 
showed, in maps and sections, as well as by descriptions*, that these 
rocks contain the fossil Sea-weed (?) Cruziana (or Bilobites), with Grapto- 
lites, Lingulae, Orthidae, and Trilobites of the genera Asaphus and Phacops 
(Calymene). He further pointed out that these Silurian masses are suc- 
ceeded by sandstones and siliceous strata probably of Devonian age, and 
the latter by limestones and other rocks, charged with fossils of the Car- 
boniferous era. Subsequently Mr. D. Forbes, correcting some errors of 
d'Orbigny, has thrown much new light upon the succession and contents 
of these Palaeozoic rocks in Chili and Peru f . 
The Silurian slates and schists form enormous bands ; and examples of them 
are well exhibited on the declivities of the plateau of Bolivia, as well as on the 
flanks of the Cordillera extending from Sorata to Illimanni. They are, in most 
* See ' Voyage dans l'Ame'rique Me'ridionale,' Lake of Titicaca, let me say that he was the first 
tome iii., Partie Ge'ologique, Paris, 1842. In person who made me acquainted with the occur- 
justice to my friend Mr. Pentland, so well known rence of Silurian Trilobites in the slaty rocks of 
to geographers by his measurements of the high this chain. 
peaks of the Peruvian Andes, around the lofty t See Quart. Journ. Geol. Soc. vol. xvii. p. 7 &c. 
