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This section is representative of the southern Pacific coast.

This section is representative of the north-west part of the country.

.—A detailed bibliography for North American geology from 1732 to 1891, inclusive, is given in U.S. Geological Survey Bulletin 127 (1896); for 1892-1900 in Bulletin 188 (1902); for 1901-1905 in ''Bull. 301 (1906); for 1906-1907 in Bull. 372 (1909); for 1908 in Bull. 409'' (1909), &c. A few of the more important and available publications are enumerated below.

General Treatises.—T. C. Chamberlin and R. D. Salisbury, Geologic Processes (New York) and Earth History (2 vols., New York); J. D. Dana, Manual of Geology (New York, 1862); W. B. Scott, Introduction to Geology (New York, 1897); and Joseph Le Conte, Elements of Geology (New York, 1878).

Official Reports.—F. V. Hayden, Reports of the U.S. Geological and Geographical Survey of the Territories (12 vols., Washington, 1873-1883); Clarence King, Geological Exploration of the Fortieth Parallel (7 vols. and atlas, Washington, 1870-1880); George M. Wheeler, Geographical and Geological Exploration and Surveys West of the 100th Meridian (7 vols. and 2 atlases, Washington, 1877-1879); and Reports of the U.S. Geological Survey (since 1880): (1) Monographs on special topics and areas, about 50 in number; (2) Professional Papers—monographic treatment of somewhat smaller areas and lesser topics, about 60 in number; (3) Bulletins, between 300 and 400 in number; and (4) Annual Reports (previous to 1903) containing many papers of importance, of the sort now published as Professional Papers. Reports of state geological surveys have been published by most of the states east of the Missouri river, and some of those farther west (California, Washington, Kansas, Nebraska and Wyoming) and south (Arkansas, Texas and Louisiana). Among the more important periodicals are the Bulletin of the Geological Society of America (Rochester, N.Y., 1889 seq.); the American Journal of Science (New Haven, Conn., 1818 seq.); the American Geologist (Minneapolis, 1888 seq.); Journal of Geology (Chicago, 1893 seq.); Economic Geology (Lancaster, Pa., 1905 seq.). Occasional articles of value are to be found in the American Naturalist and Science, and in the Transactions and Proceedings of various state and municipal academies of science, societies, &c.

 

The chief features of the climate of the United States may be best apprehended by relating them to the causes by which they are controlled. Two leading features, from which many others follow, are the intermediate value of the mean annual temperatures and the prevalence of westerly winds, with which drift the areas of high and low pressure—cyclonic and anticyclonic areas—controlling the short-lived, non-periodic weather changes. The first of these features is determined by the intermediate position of the United States between the equator and the north pole; the second by the equatorial-polar temperature contrast and the eastward rotation of the planet. Next, dependent on the inclination of the earth's axis, is the division of the planetary year into the terrestrial seasons, with winter and summer changes of temperature, wind-strength and precipitation; these seasonal changes are not of the restrained measure that is characteristic of the oceanic southern temperate zone, but of the exaggerated measure appropriate to the continental interruptions of the northern land-and-water zone, to which the term “temperate” is so generally inapplicable. The effects of the continent are already visible in the mean annual temperatures, in which the poleward temperature gradient is about twice as strong as it is on the neighbouring oceans; this being a natural effect of the immobility of the land surface, in contrast to the circulatory movement of the ocean currents, which thus lessen the temperature differences due to latitude: on the continent such differences are developed in full force. Closely associated with the effect of continental immobility are the effects dependent on the low specific heat

and the opacity of the lands, in contrast with the high specific heat and partial transparency of the ocean waters. In virtue of these physical characteristics, the air over the land becomes much warmer in summer and much colder in winter than the air over the oceans in corresponding latitudes; hence the seasonal changes of temperature in the central United States are strong; the high temperatures appropriate to the torrid zone advance northward to middle latitudes in summer, and the low temperatures appropriate to the Arctic regions descend almost to middle latitudes in winter. As a result, the isotherms of July are strongly convex poleward as they cross the United States, the isotherm of 70° sweeping up to the northern boundary in the north-west, and the heat equator leaping to the overheated deserts of the south-west, where the July mean is over 90°. Conversely, the isotherms of January are convex southward, with a monthly mean below 32° in the northern third of the interior, and of zero on the mid-northern boundary. The seasonal bending of the isotherms is, however, unsymmetrical for several reasons. The continent being interrupted on its eastern side by the Gulf of Mexico and Hudson Bay, with the Great Lakes between these two large water bodies, the northward bending of the July isotherms is most pronounced in the western part of the United States. Indeed the contrast between the moderate temperatures of the Pacific coast and the overheated areas of the next interior deserts is so great that the isotherms trend almost parallel to the coast, and are even “overturned” somewhat in southern California, where the most rapid increase of temperatures in July is found not by moving southward over the ocean toward the equator, but north-eastward over the land to the deserts of Nevada and Arizona. So strong is the displacement of the area of highest interior temperatures westward from the middle of the continent that the Gulf of California almost rivals the Red Sea as an ocean-arm under a desert-hot atmosphere. In the same midsummer month all the eastern half of the United States is included between the isotherms of 66° and 82°; the contrast between Lake Superior and the coast of the Gulf of Mexico, 1200 m. to the south, is not so great as between the coast of southern California and the desert 150 m. inland to the north-east. In January the northern water areas of the continent are frozen and snow-covered; Hudson Bay becomes unduly cold, and the greatest southward bending of the isotherms is somewhat east of the continental axis, with an extension of its effects out upon the Atlantic; but the southward bending isotherms are somewhat looped back about the unfrozen waters of the lower Great Lakes. In the midwinter month, it is the eastern half of the country that has strong temperature contrasts; the temperature gradients are twice as strong between New Orleans and Minneapolis as on the Pacific coast, and the contrast between Jacksonville, Fla., and Eastport, Me., is about the same as between San Diego, Cal., and the Aleutian Islands.

The strong changes of temperature with the seasons are indicated also by the distribution of summer maxima and winter minima; summer temperatures above 112° are known in the south-western deserts, and temperatures of 100° are sometimes carried far northward on the Great Plains by the “hot winds” nearly to the Canadian boundary; while in winter, temperatures of -40° occur along the mid-northern boundary and freezing winds sometimes sweep down to the border of the Gulf of Mexico. The temperature anomalies are also instructive: they rival those of Asia in value, though not in area, being from 15° to 20° above the mean of their latitude in the northern interior in summer, and as much below in winter. The same is almost true of the mean annual range (mean of July to mean of January), the states of the northern prairies and plains having a mean annual range of 70° and an extreme range of 135°. In this connexion the effect of the prevailing winds is very marked. The equalizing effects of a conservative ocean are brought upon the Pacific coast, where the climate is truly temperate, the mean annual range being only 10° or 12°, thus resembling western Europe; while the exaggerating effects of the continental interior are carried eastward to the Atlantic coast, where the mean annual range is 40° to 50°.

The prevailing winds respond to the stronger poleward temperature gradients of winter by rising to a higher velocity and a more frequent and severer cyclonic storminess; and to the weaker gradients of summer by relaxing to a lower velocity with fewer and weaker cyclonic storms; but furthermore the northern zone occupied by the prevailing westerlies expands as the winds strengthen in winter, and shrinks as they weaken in summer; thus the stormy westerlies, which impinge upon the north-western coast and give it plentiful rainfall all through the ear, in winter reach southern California and sweep across part of the Gulf of Mexico and Florida; it is for this reason that southern California has a rainy winter season, and that the states bordering on the Gulf of Mexico are visited in winter by occasional intensified cold winds, inappropriate to their latitude. In summer the storm westerly winds withdraw from these lower latitudes, which are then to be more associated with the trade winds. In California the effect of the strong equatorward turn of the summer winds is to produce a dry season; but in the states along the Gulf of Mexico and especially in Florida the withdrawal of the stormy westerlies in favour of the steadier trade winds (here turned somewhat toward the continental interior, as explained below) results in an increase of precipitation. The general