PLATEAU. 98 great blocks of strata ujiliftcd durinjr niountaiii foldiiif; with little ili-.tiirl)aiice of thi> layers. Thus the uplift that fonneil the Western Cordil- leras raised the plateau known as the Great Plains, whieh rises ;;enll.v from the Mississippi to an elevation of 4l)00-li()00 feet at the base of the Koekies; the Colorado Plateau, between the Roekies and the Sierra Xevada. COOO'JOOO feet high; and the Cohinihia Plateau of Oregon and Washinpton. West of the Apnalaehians. and up- lifted with them, is the Alle-;!»ny Plateau, 2000 feet or more in hcifrht, whieh extends from east- ern Xew York to Tennessee and includes hilly central Xew York, western Pennsylvania, West ^■irninia, eastern Kentucky, and Tennessee. The uplift of the Himalayas has formed the lofty plateau of Central Asia, which rises from 10.000 to 1. '5,000 feet. Plateaus are frequently so sculptured by denudation as to resemble moun- tains in form as well as in height, as in the case of the Catskills, a part of the Allegheny Plateau. Their tops, however, form an even sky line, their strata are nearly horizontal, and it is only the excessive denudation, carving deep valleys, that has given them the mountainous appearance. For such dissected plateaus the name jiseudo- mountains has been proposed. Where sculptured plateaus are adjacent to mountains it is often dilVicult to tell where the plateau leaves off and the true moiintain begins. Because of their elevation, plateau streams have great cuttiuL' ]iower and often form deep gorges, and. in arid regimis, cannns. of which the best example is the (Irand Canon of the Colo- rado. In arid regions the plateau topogra])hy is typically angular. The walls of the cafitms consist of alternate precipices and talus slopes, as resistant and weak rock layers are encoun- tered, and between the streams are Hat-topped areas, capped by resistant strata, and known in Western America as mesas if large, and as buttes if small. It is this llat-topped condition of the plateau, faced by steep slopes, that has given rise to the name tableland as a synonym for many plateaus in arid lands. Plateaus," in moist cli- mates, where the work of weathering and erosion is greater, are far less rugged and angular. By reason of their height, plateaus are generally cool, and, because of their associati<m with moun- tains, they are often arid, having rain-bearing winds cut off by the higher land near by. Plateaus are not infrequently deserts, for other reasons. The high plateau region of Africa and Arabia, for example, is traversed by the drying trade winds the year round and has little or no vegetation. Often the only parts of a desert plateau capable of sui)porting human life are the alluvial fans, on which streams (low from the adjacent mountains for a short distance before losing themselves in the sand. Portions of many plateaus, for cxami)le that of the Columbia, would be capable of cultivation if the water could be led out of the canon valleys. In temperature there is often great range, from the warm base to the cool or cold top. This is well illustrated in the plateau of Mexico, whieh rises from a tropi- cal climate to that of the cool temperate zone. DifTerence in tem[)erature is accompanied by a decrease in evaporation and an increase in pre- cipitation, so that the tops of high plateaus are often forest-covered, as in the case of the Colo- rado Plateau, which rises from the desert region of western Arizona. PLATE MARKS. When the .strata of dc('|ily dissected plateaus contain mineral deposits, like the coal of western I'cnnsylvania and West Virginia, they are im- ])ortant mineral-producing regions, because the mineral deposits are exposed where the deep valleys cut into the horizontal strata. In the Allegheny Plateau such valleys have cut down fully 1000 feet, while in the Colorado Canon the horizontal layers are levealed for over 0000 feet. Agriculture is not usually highly developed on plateaus, partlj- because of tin- climate, partly because of the ruggedncss. Grazing is the most widespread industry on arid plateaus, as in the Western United States, where large areas are oc- cupied only by scattered ranchmen. Consult: Davis, Physical Oeograj>hy (Boston, 1898); Tarr and llcllurry, Physical Geography (New York, 1897). See Mountain. PLATEAU, J0.SEP11 AxToiNE FERDiXANa (lS01-.S:i). A Belgian physicist. He was born in Brussels and was educated at Li6ge. At the foundation of the University of Ghent in 1835 he became professor of experimental physics and as- tronomy, a position he occupied until 1871. His researches were largely carried on in the lield of optics and he devised the anorlhoscope and other apparatus. His work is most remarkal)le inasmuch as much of it was accomplished while he was suffering from total blindness brought on by ex- perimental work about 1840. He invented the experiment which has since been known by his name to illustrate molecular forces, in which a mixture of alcohol and water is made of the same density as olive oil and a drop of the latter pl.aced in it. The oil will assume a spherical shape, to which it will always return. He also originated the interesting experiment in which wire frames are immersed in soajjy water and then films produced which intersect in .straight, .sharp lines. These and other experiments are described in fitatifjue des liquides soiimis aux forces mol6- cuhiircs, published in 1873. Professor Plateau also prepared a bibliography of the subjective ])henoraena of vision to the end of the eighteenth century. PLATED WARE. See Tableware, Silver- Pl.ted. PLATE MARKS, or Hall IVIarks. The legal impressions made in Great Britain on watch- cases, jewelry, and gold and silver plate, after the material has been assayed at the various Govern- ment assay offices. In the case of jewelr.y the marks are, with a few exceptions, optional. There are four distinct kinds of plate marks: (1) the standard or quality mark, (2) the mark of a par- ticular office at which the article was assayed, (3) the mark by which the date of marking may be ascertained, and (4) the maker's private mark. Formerly if the article was dutiable the head of the reigning sovereign was also added, but in 1798 watch-cases were exempted from this rule, and in 1890 the practice was entirely abandoned. The standard mark of the London office is a lion passant ["^^1 f'"" terling silver; a crown with the figures 22 ['@]2Z ^'"" 22-carat gold; and the crown with the figures IS for 18-carat gold. Up to 1845 a lion passant was likewise the standard mark of 22-carat gold, and from 1G97 to 1723, during which time the quality of stand- ard silver was raised from .925 to .959, the
