424                                                  SILURIA.                                  [Chap. XVIII.
 

CHAPTEE XVIII.
SUCCESSION OF PRIMEVAL ROCKS IN AMERICA.

ORDEE OF THE PALEOZOIC ROCKS IN SOUTH AJVIERICA (tHE ANDES), THE UNITED STATES,

AND BRITISH NORTH AMERICA.

The researches of geologists have demonstrated that there was a wide
diifusion of simUar groups of animals over the globe during the Primeval
Periods. A striking proof of this fact is, that many of the Palaeozoic fossUs
which we have foUowed 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 foUowed by
a similar discovery in Europe. The highly important researches of Logan
and his associates, demonstrating that aU the Palaeozoic rocks, so weU de¬
scribed in the United States by HaU 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 North 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 deUneated
by that great traveller, in whose youthful days fossUs were Uttle studied,
have since been referred, by means of their organic remains, to the Silurian
System. FoUowing up the inquiries of his precursor, Alcide d'Orbigny
showed, in maps and sections, as weU as by descriptions*, that these
rocks contain the fossU Sea-weed (?) Cruziana (or Bilobites), with Grapto¬
lites, Lingulae, Orthidae, and TrUobites of the genera Asaphus and Phacops
(Calymene). He further pointed out that these SUurian masses are suc¬
ceeded by sandstones and sihceous 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 Ught upon the succession and contents
of these Palaeozoic rocks in ChiU 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 rAmdrique Mdridionale,'     Lake of Titicaca, let me say that he was the flrst

tome iii,, Partie   Geologique,  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.