Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/721

Rh GEOLOGY.] AMERICA 679 area of United States territory suitable for cotton planta tions and for slave labour was so far limited, and confined to the shores of the Gulf of Mexico and the Atlantic sea board. At the same time, in the latitudes north of these, between 40 and 50, the whole vast prairie region from the Ohio to the Rocky Mountains was adapted to the growth of corn, while it had such continuity and unifor mity of surface, and was so pervaded by the grand river- system of this middle plain of North America, that it could not well undergo political division. The entire West being thus secured to the Union, and the better part of the South being thus rescued from the curse of Negro slavery, by the moulding hand of nature in planning the distribution of mountain ranges and the level ground all over this continent, we may consider that the political and social destinies of the great English Republic, vindi cated in the civil war from 1861 to 1865, were predeter mined in the formation of the land. eology. The North American continent affords an interesting study of the geological changes and adjustments, by which the mighty work of preparation for what promises to be a noble development of humanity was slowly effected in the remote epochs of the past. The oldest sedimentary rocks anywhere found on the globe are those which under lie the whole of Canada, New Brunswick, and Newfound land, the Labrador peninsula, and the country north of Lake Superior, perhaps also the less explored regions of the far north-west towards the Arctic Sea. This series, named the Laurentian, from the St Lawrence river, is per ceived to exist in Europe only in a few scattered instances, in the Hebrides, and in Norway or Sweden. In North America it occupies the most extensive areas; the thick ness of its beds is estimated by Sir William Logan at 30,000 feet; it rises to hills or mountains 4000 feet high, and in the deep gorge of the Saguenay river, forms per pendicular cliffs of 1500 feet. Only one fossil animal, which has been called the Eozoon Canadense, has been dis covered to have left its trace in this most ancient bottom of the primeval ocean; it was one of the Foraminifera, which covered its gelatinous body with a thin crust of carbonate of lime, having numerous holes or pores for the emission of its filament-members, with which to feel and to feed outside. Next to the Laurentian, but with a vast unknown interval of time, comes the Huronian or Lower Cambrian series. It is suggested by geologists that, as the vast level bed of the Laurentian sea was cracked by internal changes of the earth s density, these cracks threw up certain ridges along the surface of the present con tinent, which laid a foundation for the principal mountain ranges we have described. At the borders and extremities of these mountain ranges, it is evident that there were intense volcanic eruptions, producing great quantities of lava and conglomerate, basalt, greenstone, and other formations resulting from igneous action. The northern shores of Lakes Huron and Superior exhibit results of this kind ; but it is in the table-lands between the Rocky Mountains and the west coast ranges, as in the singular lava beds near the Klamath, on the frontier of Oregon and North California, that volcanic forces have made their strongest marks on the earth. On the eastern side also of the grand Cordillera, between the sources of the Missouri and of the Yellowstone rivers, is a wonderful region of boiling springs or geysers, of sulphur beds and other natural curiosities, which have recently been described by Dr Hay den, of the United States Government Survey. To speak more generally of the local arrangement of dif ferent geological formations, it may be remarked that crystalline rocks are spread over the western parts of North America, from Alaska to Nicaragua, and over the most northern parts, also including Greenland; but some of later date are found in the eastern or Appalachian range, consisting of felspathic gneiss and quartz rocks, mingled with talcose and chloritic schists. The Palaeozoic forma tions occupy that middle part of the continent which lies between the Rocky Mountains and the great lakes, as well as the shores of Hudson s Bay, and some portions of the Atlantic coasts. With reference to the Lower Silurian or Siluro-Cambrian period, it is abundantly illustrated by the Trenton and associated limestones, which can be traced over 40 of longitude, their beds consisting entirely of debris of coral, shells, and crinoids, from the shallow inland sea, teeming with animal life, that once filled the whole level space between the Alleghanies and the Rocky Mountains, separated by these ridges from the Atlantic and Pacific Oceans, and sheltered from the cold Arctic currents by the northern Laurentian highlands. This space, which is now the central plain of North America, comprising the prairies and the Mississippi valley, was then a coral sea with archipelagos of volcanic isles, resembling that of Australasia in the South Pacific. The next forma tion succeeding the Silurian presents immense deposits of sandstone and shale from the muddy waters troubled by subterranean motions ; this is the Devonian formation, which in America has been called the Erian, on account of the great development of such beds south of Lake Erie. The deposits of this period in the western continent are stated to be 15,000 feet in thickness. They include the cliff limestones, studded with calcareous corals of great size and beauty of shape, noticed by Sir Charles Lyell at the falls of the Ohio, near Louisrille; these limestones are estimated to extend, as an ancient coral reef, over 500,000 square miles of the American middle plain. In the State of New York and in Western Canada there is the corni- ferous limestone, in which the imbedded corals have been replaced, in the cavities they once filled, by flinty horn- stones which present the perfect coral forms, as though cast in a mould. In the Carboniferous age the great internal sea of the continent was slowly changed into swampy flats and shallow lakes or creeks, and gradually filled with a rank vegetable growth, afterwards buried under later marine deposits and pressed into the existing coal-beds. Of this period there are very extensive de velopments throughout the eastern half of the great middle plain to the Alleghanies. This portion of America seems to have been land, covered with the forests of that period, while the western half of the middle plain, a northward extension of the Gulf of Mexico, was still under water. As the eastern half of North America, between the Mis sissippi and the Atlantic, was thus in the Carboniferous era well raised out of the sea, it exhibits no traces of the succeeding Permian age, such as we find in Europe. The earliest periods also of Mesozoic time have failed to leave any record here, but their formations appear towards the western range of mountains in what was the bed of a Mediterranean Sea. It is, however, the Cretaceous system, with its characteristic green sand, its sands, clays, rnarls, and soft grey limestones, that occupies most space in Western America, between the meridians of 97 and 112. These strata, overlaid sometimes by those of the Tertiary periods, extend through the country up the Missouri, the Platte, the Arkansas, and the Red River, to the Rocky Mountains; they also form parts of the plains enclosed by different mountain ranges beyond the Cordillera. Along the eastern side of the Appalachians there is a broad belt of the Cretaceous formation, stretching from the Delaware across the upper parts of Virginia, Carolina, Georgia, and Alabama. Among the fossil animal remains discovered in this formation in North America, which are enumerated in a separate list, one of the most remarkable is that of the Mosasaurus, a combination of the serpent with the lizard