Chap. XVIII.] SILUKIAN KOCKS IN THE UNITED STATES. 
433 
the consideration of these data we shall return in the last Chapter, in explaining 
the conditions under which some of the oldest marine deposits were formed. 
Notice must also here be taken of the survey of the extensive territories of 
Wisconsin, Iowa, and Minnesota, by which the late Dr. Dale Owen has extended 
true Palaeozoic classification on a very broad scale. The area examined by this 
author and his associates, Drs. Norwood and Shumard, is much larger than Great 
Britain ; and by far the greater portion of its subsoil is referred by them to the 
Silurian, Devonian, and Carboniferous rocks *. This magnificent region is tra- 
versed by the Mississippi in its descent from the boundaries of British America 
and Lake Superior, and is included between that mighty river and its huge 
affluent the Missouri. Surrounding and supporting an enormous Carboniferous 
tract, the older primeval rocks rise out successively in very slightly inclined 
positions. The results of this survey which have most interested me are the 
discovery of two bands containing Trilobites, in the lowest Silurian sandstone, 
and the fact that the organic forms of these beds are considered to be referable 
to the ' Zone Primordiale ' of Barrande as it exists in Bohemia and Scandinavia, 
or in the ' Lingula-flags ' of Britain (see pp. 41, 350, 372). We thus obtain 
proofs that the earliest Silurian deposit of America, whether in the condition of 
sandstone at Potsdam and in the region of the Mississippi, or as the meta- 
morphic and slaty rock of the eastern seaboard of the United States, does really 
contain, as in Europe, the first clearly recognizable fossil animals which pertain 
to this series of life f. 
Specimens of a very large species of Paradoxides have been found by 
Professor W. Rogers in a band of siliceous and argillaceous slate, ten miles to 
the south of Boston, which is enclosed between great masses of igneous rocks. 
A longer and broader species of the same genus has been sent from St. John's, 
Newfoundland, to the Bristol Institution ; and a species of Conocephalus has 
been found in the Sandstone of Georgia %. A large portion of the crystalline 
masses which flank the coal-fields of the British Colonies of New Brunswick, 
Nova Scotia, &c, have, also proved to be metamorphosed Silurian strata, thus 
largely extending the structural analogy between North America and the 
Highlands of Scotland. 
The Lower Silurian, so termed and laid down on their map by Dale Owen and 
his associates, consists, in its inferior portion, of pebble-beds, grits, and sand- 
stone § of red and greenish colours, which, as they range upwards, begin to 
alternate with magnesian limestone. Then, in ascending order, the limestones 
with green grains (resembling some of the Russian Lower Silurian, p. 355) pre- 
dominate, and are in parts oolitic, in other parts quartzose and cherty. Again, 
sandstones, usually white, recur, and are surmounted by shelly beds which, in 
Iowa and Wisconsin, represent the Trenton Limestone of New York and the 
Llandeilo formation of Britain. 
The Upper Silurian rocks are poorly represented, in these unfrequented tracts, 
by what Dr. Dale Owen termed the ' Upper Magnesian Limestone/ consisting of 
* This work, following so many other valuable stratum in America (the Potsdam Sandstone), are 
publications, is a proof that the Senate, as well as termed, by Dr. Owen, Dikelocephalus, Loncho- 
the State Governments, has justly appreciated cephalus, &c. Judging from the drawings, these 
the value of the application of geological science genera seem to be referable to the same group as 
as a prelude to the settlement of a new country. Paradoxides. They are associated with Fucoids 
t See ' Proofs of the Protozoic Age of some of and Lingulae. After studying the work of Dr. 
the Altered Eocks of Eastern Massachusetts,' by Dale Owen, M. Barrande is convinced that the 
Professor W. B. Eogers, Proc. American Academy author has established the proof of the existence in 
of Arts and Sciencies, vol. iii. 1856; and Proc. the New World of the ' Primordial ' Silurian zone 
Boston Nat. Hist. Soc. vol. vi. pp. 27, 40, &c. of Bohemia and Sweden, and the same order of 
I See Mr. Salter's account of these fossils, succession upwards from it into the mass of the 
Quart. Journ. G-eol. Soc. vol. xv. p. 551, &c. Silurian strata as in Europe. In this view M. de 
§ The Trilobites of this, the lowest Silurian Verneuil coincides. 
2p 
