PENNSYLVANIA, popularly known as the "Keystone" State because of its central position among the original 13 Col onies, is one of the Commonwealths of the United States of America, lying mostly between lat. 43' (the Mason and Dixon line) and 42° N. and long. 43' and 8o° 31' W. of Greenwich. The State is in the form of a rectangle, except in the north-west where a triangular projection, extending to 42° 15' N. lat., gives it a shore-line of almost 4o m. on Lake Erie, and in the east where the Delaware river with two large bends makes deep indentations into New York and New Jersey. Pennsylvania is bounded north by New York; east by New York and New Jersey, from which it is separated by the Delaware river; south by Delaware, Mary land and West Virginia, and west by the "Panhandle" of West Virginia and by Ohio. The total area is 45,126 sq.m., of which 294 sq.m. are water surface.
Physical Features.—Pennsylvania skirts the coastal plain in the south-east below Philadelphia and is traversed from north east to south-west by the three divisions of the Appalachian province—Piedmont or older Appalachian belt, younger Appala chian ridges and valleys, and the Allegheny plateau. In the north-west corner is a small part of the Erie plain. The entire surface has a mean elevation of about f,foo ft. above the sea, varying from sea level on the Delaware river to 3,212 ft. (Negro mountain) in Somerset county. On the north and west borders of the lowland extending along the Delaware are two parts of a chain of semi-detached and usually rounded hills, known as the South mountains. The north-east part extends from New Jersey through Easton to Reading. The south-west part is a north eastern prolongation of the Virginia Piedmont, known as the Cumberland prong. The Pennsylvania portion of the younger Appalachian ridges and valleys, known as the central province of the State, embraces the region between the South mountains on the south-east, and the crest of the Allegheny plateau or Alle gheny front on the north-west. It extends from south-west to north-east about 23o m. and has a nearly uniform width of so m., except that it narrows rapidly as it approaches the north-east cor ner of the State. The crest lines are often of nearly uniform height for miles and generally are little broken except by an occasional V-shaped wind gap, a narrow water gap or a rounded knob. The valleys, except the Appalachian or Great valley, rarely exceed more than a few miles in width. The Pocono plateau, into which the central province merges at its north-east extremity, is a continuation of the Catskill plateau southward from New York.
It is little broken, except by shallow valleys and occasional knobs. The Allegheny plateau, which extends from the crest of the Alle gheny front to and beyond the west and north borders of Penn sylvania and covers more than one-half of the State, is much dissected and contains the highest elevations within the State.
The Pocono plateau in the north-east, nearly all of the central and south-east provinces and the north-east portion of the Alle gheny plateau in the north-west are drained by the Susquehanna and Delaware river systems into the Chesapeake and Delaware bays. The greater part of the Allegheny plateau is drained by the Allegheny and Monongahela rivers into the Ohio river. The extreme southern portion of the central province and the ex treme western portion of the south-east province are drained by tributaries of the Potomac; the Erie plain is drained by short streams into Lake Erie; and a very small section of the Allegheny plateau, in the northern part of Potter county, is drained by the Genesee river into Lake Ontario. The Susque hanna is a wide and shallow stream with a zigzag course and numerous islands, but both the Susquehanna and the Delaware, together with their principal tributaries, flow for the most part transverse to the geological structure. In the gorges and water gaps through which they pass in the mountain region is some of the most picturesque scenery in the State. These gorges, too, are of great economic importance as passages for railways and high ways. The lower portion of the Delaware river has been entered by the sea as the result of the depression of the land, giving a harbour, at the head of which developed the city of Philadelphia. Climate and Soil.—The temperature of Pennsylvania is mild and equable in the south-east province where the ocean influences it and where the mountains bounding it on the north and north west are some protection from the colder winds. The crests of the higher ridges in the central province are delightfully cool in summer, but the adjacent valleys are subject to excessive heat. In winter the mountain valleys may experience severe cold. The mean annual temperature is about 52° F. in the south-east, 50° in the centre, in the north-west and 49° on the shore of Lake Erie. The summer maxima on the mountains are usually 8° to 10° less than in the valleys directly below them. There are many summer resorts in the mountains. The average annual rainfall is 44 inches.