sissippi, to the Gulf of Mexico. These lines of elevation and depression have had a powerful influence on the physical structure of the country, and in future ages are probably destined to exercise one equally conspicuous on the distribution of the population, with its civil and social relations. The same causes have determined the geographical distribution of the mineral masses and the grand geological features of the country. The power which elevated the Alleghany range has also raised the whole central basin, with the recent formations it contains, so that the older and metamorphic strata appear only in the mountains, and on their slope towards the sea. There is thus a band of primary rocks, which, commencing in Maine and the New England States, where it covers almost the whole country, extends south to the Hudson. Beyond this, with a short interruption, it forms another band, eighty to a hundred miles wide, as far south as the Alabama river, where it is lost below the alluvium of the Mississippi. West of this river, the Rocky Mountains consist of similar primary formations, broken through by volcanic eruptions, which, though unknown in the east, are here said to cover a vast extent of territory. It would appear that primary rocks also connect this range with those in the upper part of New York; almost enclosing on three sides the central basin, in which is contained an immense deposit of secondary and more recent strata, which are also found in considerable abundance along the Atlantic coast. The general structure of the country is thus very simple; and we shall now notice the principal formations, beginning with the lowest or primary. As already mentioned, this covers, with few exceptions, all the northern states. In New Jersey, it is interrupted by a band of red sandstone, running south to Virginia and North Carolina; but the primary rocks appear again at Trenton, and thence form the whole of the eastern system of mountains. Their limit, on the east, is marked by a series of falls or rapids in the rivers, which are precipitated over the ledge formed by their outcrop. As the tide cannot ascend higher, many of the large cities are situated near this boundary. The falls marking it are seen near Trenton on the Delaware, Philadelphia on the Schuylkill, near Baltimore, at Georgetown on the Potomac, Richmond on James River, Munford Falls on the Roanoke, east of Raleigh, near Camden on the Pedee River, near Milledgeville on the Oconee, whence the boundary curves round to the Mississippi, near Natchez. In the latter part of their course they contract rapidly in breadth, especially in Georgia, where they leave the Atlantic basin. This formation consists of igneous and metamorphic rocks, similar to those in our own country. Of the former, granite-sometimes porphyritic or graphic, at others sienitic-is the most common, though seldom forming large masses. It has sometimes an irregular, crystalline structure, dividing into large rhomboidal fragments. In South Carolina and Georgia, the whole rock, to a depth of thirty or forty feet, is often decomposed into separate crystals of quartz, felspar, and mica, almost without cohesion. In New England, a sienitic variety, in which the mica is replaced by hornblende, is largely used for architectural purposes. This forms a transition from the true granite to the porphyries and greenstones connected with it, which are also very abundant, the igneous rocks in Massachusetts covering nearly a fourth of the surface. Of these, there seems to have been very many eruptions, Professor Hitchcock finding no less than eleven systems of veins in one place near Salem. Serpentine, usually classed as igneous, is, according to this observer, often stratified and associated with talcose slate. It is very abundant, and furnishes, in several places, a beautiful ornamental stone, named verd-antique marble. More certainly stratified is gneiss, in many varieties and great extent, forming perhaps near half the formation. Along with it micaslate is very common, less so hornblende-slate, clay slate, and talc or chlorite slate, the last rare in the gneiss. Primary or granular limestone is also abundant. In Essex county, west of Lake Champlain, Professor Emmons describes a large formation of a granitic rock, composed of blue or green Labrador felspar and hypersthene, which takes a beautiful polish, and, like the whole primary formation, contains numerous beds and veins of iron-ore. The granite in this and St Lawrence county, he also states to contain genuine injected veins of limestone; and hence, with some European geologists, he proposes to class this rock among the unstratified or igneous.* The whole north-western border of the primary rocks, from Alabama to Canada, presents a continuous belt of this rock, in general more or less magnesian. As this mineral prevails, the marks of stratification become more indistinct, till they almost disappear in the pure dolomite, which is most abundant near faults, igneous rocks, and the oldest gneiss. These facts show that the limestones are merely metamorphic deposits, altered by heat and sublimation from below, and not, as Rozet and Emmons suppose, original plutonic formations. The carbon found in the mica and tale slates, near the limestone, is also a result of this high temperature, perhaps produced by the decomposition of the carbonic acid of the calcareous rock. Throughout the whole range of the mountains, from Tennessee to New England, these rocks seem to have been elevated, folded together, and in some degree inverted. The general dip of the strata is at a high angle to the east, yet the most recent beds are found on the west, apparently covered by the older members of the series. Professor Hitchcock and others suppose that the strata, whilst yet flexible, have been compressed by powerful forces at the extremities, and consequently folded together, but other explanations have been given. Be the cause what it may, it has occasioned considerable *New York Reports for 1837-1838, p. 196. difficulty in tracing the relations of this and the next formation.* The principal deposit of gold in the United States occurs in these primary rocks, between the river Rappahannock, in Virginia, and the Coosa, in Alabama; but it has also been found as far north as Somerset in Vermont, and south to the Gulf of Mexico. Its original locality is in veins of porous quartz traversing the talcslate, and more rarely the gneiss and mica-slate, but most is procured from the stream mines in the debris of these rocks. The gravel is washed by negroes, who each collect, in general, from one to five dwts. a-day, though in some instances even 120 dwts. have been procured. From 1823 to 1836, the gold sent from this district to the mint of the United States amounted to 4,377,500 dollars, and the actual produce was estimated at twice this amount. Remains of arrow-heads and other instruments are found buried in these deposits.+ Above this is an immense continuous formation, corresponding to the transition and older secondary rocks of England. These form all the ranges of the Alleghanies, except the eastern primary system, and all the interior of the country, covering, in the United States and British America, an area 2000 miles long by 1200 wide, or two and a half million square miles. The amount of strata is no less enormous, those below the coal having been estimated as five and a half miles thick ; and in Pennsylvania Professor Rogers makes them, including this, two miles more, or 40,000 feet in all. It is thus difficult to give a general view of this immense deposit, especially as much confusion, both in the nomenclature and arrangement, prevails among the American writers. The old classification of Professor Eaton is now seldom used, and more recent geologists are en Hitchcock's Geology (2d Edition, New York, 1841), pp. 36, 37. Trans. of Penns. Geol. Soc., vol. i. p. 147. This may be found in Silliman's Journal, vol. xiv.; or somewhat modified, ib., vol. xxxvi. deavouring to establish a parallel with the English formations described by Mr Murchison. Mr Conrad affirms that this classification applies, in a more clear and satisfactory manner, to the rocks of America than to those of Europe, since the series is certainly more complete, and the organic remains more abundant in species. He also affirms, that "the inhabitants of the seas (in which these rocks were deposited) have been destroyed, and new creatures succeeded at five different epochs, and one of these groups is no more to be compared with another than is the oolite with the green sand formation.” The lowest beds of this formation are a series of graywacke, clay-slate, and limestone rocks, which Mr Conrad names the Silurian System, and classes in four divisions:-1. Hudson's slates; 2. Calciferous and Potsdam sandstones; 3. Mohawk limestone; 4. Sparry limestone. These are spread over the greater portion of New York, Ohio, Indiana, Kentucky, and Tennessee, terminating in the hilly region of Northern Alabama, where some of the New York deposits have been recognised. Mr Lyell, however, considers the Mohawk limestone and the associated rocks, seen on the shore of Lake Ontario, as older than the lower silurian beds in England. The next beds are the Ontario and Protean groups of Hall, the former consisting of red marls and sandstone, the latter of dark shale with graptolites, and limestone full of the Pentamerus oblongus and P. lævis. These groups, according to Conrad, represent the Caradoc sandstones of Murchison; but Lyell regards them as equivalent to the lower silurian rocks of that author. * Silliman's Jour., vol. xxxv. p. 246. New York Reports for 1839, p. 200. In vol. xxxviii. will be found his arrangement of the formations, twenty-four in number, below the coal, with the corresponding English rocks. The fossil species common to the Silurian rocks of Wales and the United States are, in the Caradoc sandstone, six shells and two trilobites; in the Wenlock shale, four shells and one trilobite; in the Wenlock limestone, eight corals, nine shells, and two trilobites; and in the Ludlow rocks, four shells; or, in all, eight corals, twenty-three shells, and five trilobites, that is, thirty-six fossil species. |