426 
SILUEIA. 
[Chap. XVIII. 
gical Survey of Canada/ 1865. The relations of all the Palaeozoic rocks, from 
that fundamental gneiss upwards, are therein so clearly developed, even upon a 
small scale, as to render that map worthy of the admiration of every geologist ; 
for in it twenty-five divisions of those ancient rocks are described with clearness 
and precision *. 
Like the venerable Scottish gneiss, the Laurentian of North America is 
also unconformably surmounted by hard sandstones, conglomerates, and schists 
(with limestone and chert), which, attaining the vertical dimensions of about 
12,000 feet on the north shore of Lake Huron, have been termed ' Huronian.' 
This Huronian group or system which has been paralleled by Logan with the 
Cambrian or Longmynd rocks of Britain, is covered transgressively by the lowest 
of the fossiliferous bands of the Lower Silurian or Potsdam sandstone (c 1 of 
Section, p. 169). 
For the present we may leave the consideration of those other crystalline 
rocks which form a broad and long zone, ranging from S.S.W. to N.N.E., along 
the seaboard of the United States, and extend into Canada and the British 
Colonies of Nova Scotia, Newfoundland, and New Brunswick. Now, al- 
though in geological maps of North America t these rocks have been hitherto 
laid down under the same colour as those of the Laurentian system, they are 
essentially distinct from the old gneiss and its associates, and are, as will pre- 
sently be shown (at least in several parts), nothing more than highly meta- 
morphosed Lower Silurian strata, thus affording another analogy to the rock- 
masses of the north-western Highlands of Scotland already described (p. 163 
et seq.). 
It is on the western flank of the Appalachian chain, and to the south, north, 
and west of the Laurentide Mountains, that the Palaeozoic formations are best 
exhibited. They consist of Lower and Upper Silurian, Devonian, and Carbo- 
niferous rocks, and are repeated in broad undulations, forming basins of a gran- 
deur and extent unknown, as yet, in any other part of the world excepting 
Russia. In this way they occupy large portions of the southern and western 
States — Alabama, Tennessee, Arkansas, Missouri, Illinois, Indiana, Iowa, Min- 
nesota, and Wisconsin. Such deposits range, too, along the southern side of the 
great chain of lakes, and are spread over extensive tracts of New York, Penn- 
sylvania, Maryland, and Virginia, where their stratigraphical features have been 
elaborately worked out, and] their organic remains described, by many distin- 
guished American geologists. 
These authorities have honoured my labours by including the lowest of 
these great masses of fossiliferous rocks in the Silurian system j whilst all my 
cotemporaries % who have gone from Europe to explore the United States, or 
* This work is the result of the labours of the and of Wineonsin, &c, Washington, 1835-36 [my 
Geological Surveyors of Canada, namely Logan, old friend Mr. F. first made known to me the ex- 
A. Murray, T. Sterry Hunt, and E. Billings, as- istence of the Silurian series in the United States], 
sisted, as regards the adjacent parts of the United Lyell, Travels in North America, with general 
States, by Professor James Hall, and in the ad- map, 1841-42 : London, 1843. Lyell's Geological 
jacent British Colonies by Principal Dawson, Pro- Manual, 3rd ed. p. 351. Castelnau, Systeme Silu- 
fessors James Eobb, J. B. Jukes, and others. rien de l'Amerique Septentrionale. Paris, 1843. 
t The maps of Lyell, Marcou, and others, which De Verneuil, Bulletin de la Society Ge'ologique 
have represented these eastern crystalline stra- de France, 2nd ser. vol. iv. 1847. Eichardson, 
tified rocks, with their various intrusive or igneous Narrative of an Arctic Searching Expedition, 2 
rocks, whether granite, porphyry, or greenstone, vols. London, 1851. Logan, Geological Survey 
&c, under the same Primary colour as the Lau- of Canada ; Reports to Legislative Assembly, and 
rentide Mountains, must now undergo great mo- maps. Logan and Salter on the Kocks of Lower 
diflcation. The statement made in the first edi- Canada, Brit. Assoc. Keports, 1852, p. 59 ; Quart, 
tion of this work (p. 410) is also incorrect. Journ. Geol. Soc. Lond. vols. ix. &c. Ferdinand 
See Bigsby, Trans. Geol. Soc. Lond. ser. 2. RSmer, Texas und die physischen Verhaltnisse 
vol. i., and Quart. Journ. Geol. Soc. vols, viii., des Landes: Bonn, 1849, with geological map. 
xiv., xv., &c. Featherstonhaugh, Keports on the Desor, Bull. Soc. Geol. Fr. ser. 2. vol. ix. p. 342. 
Countries between the Missouri and Red Rivers, Jules Marcou, Geological Map and Description 
