SECTION II: PHYSICAL GEOGRAPHY AND STATISTICS (cont.)
Part 17. Geology of the Mississippi Valley
The area enclosed between the Appalachians and the Cordilleras, extending to upwards of 1,500,000 square miles, the drainage basin of the Great Lakes and St Lawrence on the north and of the grand Missouri-Mississippi river-system on the south, cannot here be discussed in detail from the topographical point of view. The general features do not present the diversity seen in the regions already considered. All that can be done here is to indicate the salient points of the geology.
The belt of Tertiary and Cretaceous rocks already mentioned as forming the Atlantic slope extends, with very similar characters, curving broadly around the southern end of the Appalachians, and continuing along the Gulf and up the Mississippi valley, to about the junction of that river with the Ohio. About half of Alabama and Arkansas, all Mississippi and Louisiana, parts of Tennessee and Kentucky, and a very small corner of Missouri are underlain by these newer formations. Nearly the whole of Texas is similarly situated with respect to its geology. In the northern central portion of the last-mentioned State the marly and gypsiferous red sandstones of Triassic age cover a large area, bordered on the southeast by a little-known coal-field, or carboniferous age, with a very small patch of Azoic or Archaean rocks at its southern termination almost exactly in the center of the State. Tracing the geological formations northward from Texas into New Mexico and along the eastern flank of the Rocky Mountains, we find the belt of Cretaceous and tertiary covering a very large area, extending as far east from the mountains as the center of Kansas, and covering nearly all Nebraska and Dakota, the north-western corner of Iowa, and the western half of Minnesota. The Triassic belt mentioned as occurring in Texas occupies a broad area in the Indian Territory and the southern central part of Kansas. It is also quite extensively exposed along the streams of New Mexico, forming the border of the Llano Estacado or Staked Plain. The Cretaceous and Tertiary rocks of the west have nowhere anything like the economical importance which they have in New Kersey; but from a palaeontological point of view they are of interest, and, especially in the lower Mississippi valley, have been studied with care and in considerable detail by the State geologies. Included within this border of more recent rocks, and comprising the whole of the North- Eastern Central group of States (see below), as well as the western portion of the North-Western Central, and smaller portions of the South-Eastern and South-Western Central groups, is a region underlain almost exclusively by Palaeozoic rocks, covered with post-Tertiary and recent detrital formations, the intermediate members of the geological series being entirely wanting. These Palaeozoic strata include very extensive and complete representations of both the Lower and Upper Silurian series, and also of the carboniferous, including both the upper and lower members of this division of the Palaeozoic. As we leave the Alleghany escarpment in going westward we find the disturbances of the strata becoming less and less marked, what flexures there are being exceedingly broad, so that over large areas the rocks seem to lie in an almost undisturbed horizontal position. the geographical distribution of the areas underlain by the Coal-measure in this region is indicated below (p. 812). Calcareous and calcareo-magnesian formation are especially prominent over this great area of nearly undisturbed strata. As we proceed westwards from the Appalachian belt we find the purely detrital and siliceous rocks diminishing and the calcareous gaining in importance and thickness. Thus the millstone grit, which on its eastern edge is in places more than 1000 feet thick, is found in parts of the Mississippi valley to have diminished to a few feet or even in places to have disappeared altogether. With this diminution of coarser detrital and siliceous comes in a wealth of organic forms, and the rocks of the region in question have been most fruitful of material for the palaeontologist. Towards the western and north-western portions of the Palaeozoic area there occur several marked breaks in the uniformity of the geological character of the region. These are due to the appearance at the surface of rocks older than the lowest Silurian-rocks, indeed,, which, up to the present time, in spite of forty years of diligent search, have not been found to exhibit any traces of life. For this reason these rocks, which unconformably underlie the Lower Silurian, and are in such a position as to prove beyond a doubt that they assumed that position before the deposition of the lowest known fossiliferous rocks, were called Azoic by Foster and Whitney, but are now more generally known as Archaean, a name substituted by Dana. The Azoic areas of central Texas, northern Texas, and central Arkansas are comparatively small, and have been but little studied in detail, since, thus far, they have not been shown to be of much economical value. The Azoic area in south-eastern Missouri is also of small dimensions, but economically important, since iron ores, large in quantity and of great purity, occur here, at the well-known Iron Mountain, Pilot Knob, and other localities. Far more important than those already mentioned, however, is the Azoic area of northern Wisconsin and north-western Minnesota, which is in direct connection with the great Azoic district of so much importance in Canada as forming the mass of the Laurentian mountains. The region in Wisconsin forming the divide between the waters flowing into Lake Superior and those uniting with the Mississippi is one of Azoic rocks, and from this a long spur extends south-westernly through Minnesota and north-easterly to Lake Superior. It is in this region and in this formation that the iron mines occur which are of so much importance to the country (see p. 814), the principal mines lying about 1500 feet above the sea or 900 feet above Lake Superior. To the north-west of this Azoic area, on the borders of the lake, is the very important copper region (p. 816). The copper-bearing range, which rises in places to an elevation of as much as 2000 feet above the sea-level, is made up of old volcanic masses interstratified with sandstones and conglomerates of Lower Silurian age. The so-called trappean range runs from the extremity of Keweenaw Point south-westerly along and near the shore of the lake, and finally disappears some distance beyond its western end; but the portion of the range which is of importance for its copper mines is in Michigan and on or near Keweenaw Point. The detrital formations which cover most of the surface of the Palaeozoic area, the boundaries of which have here been indicated, are of varied character. Over much of the country the principal detrital material present is that which has been left behind by the slow wasting away, under the influence of the rain and other atmospheric agencies, of the calcareous rocks which there occur. This kind of material forms the bulk of the soil in the higher portions of the region lying near the Ohio and its junction with the Mississippi, and north-west to Minnesota. The river-bottoms grow wider as we proceed in the direction of the drainage towards the Gulf of Mexico, but the thickness of alluvial soil overlying the Tertiary and Cretaceous does not seem, in general, to be very great. The material liberated by the decomposition of the rock has been so fine that most of it has been easily carried away where the volume of water in the rivers was considerable. Coarser detritus occurs near the mountain ranges, especially those on the eat, where strata made up in large part of pebbles or even boulders of quartzose or other indecomposable rocks form a considerable portion of the underlying formations. An important feature in the surface geology of the northern portion of the central area as well as of the extreme north-eastern portion of the United States, or that comprised within New England, New York, the northern part of Pennsylvania, and the region adjacent to and south of the Great Lakes, is the presence of a large amount of coarse detrital material in the form of boulders, gravel, and sand, which has been, in large part, brought from the north, and which is mixed very unequally in different regions with the material resulting from the disaggregation, decomposition, and abrasion of the closely adjacent or underlying rocks. The origin and mode of distribution of this so-called "northern drift" has long been a fruitful subject of discussion among American geologists. By far the larger number of those who, in later years, have discussed among American geologist. By far the larger number of those who, in later years, have discussed the problem have been inclined to ascribe the origin of the drift almost entirely to glacial causes. It is assumed that the northern portion of the continent was during the so-called "glacial epoch," covered deeply with ice, and that all, or nearly all, that we see at the present time upon the surface of the region thus covered is the result either of this icy envelope or of the floods produced by its melting. The present writer believes the phenomena to be much more complicated and difficult of explanation than is generally supposed, but contents himself with simply starting what is the current belief among American geologists.
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