Page:Encyclopædia Britannica, Ninth Edition, v. 18.djvu/526

500 PENNSYLVANIA in spring. The average annual rainfall ranges from 36 inches in the western counties to 42 inches at Philadelphia. Destructive "freshets" descend the eastern rivers when the ice breaks up ; for the Delaware and Susquehanna rivers are almost every year frozen over from tide -water to their sources ; thunderstorms happen in the midst of winter ; the January thaw is always to be apprehended ; and when heavy rains break up the ice and it accumulates in the gaps of the mountains, the main river -channels become scenes of inevitable disaster. In 1837 the valley of the Lehigh was swept clean for 60 miles, the dams and locks of the canal were all destroyed, and every bridge and mill disappeared. Along the lower Susquehanna the floating ice has often been piled upon the railroad embank ment to the height of several yards. Even in midsummer a heavier downpour than usual in 1836 carried destruction through the valley of the Juniata. But the affluents of the Ohio river in the western part of the State are subject every year to this danger. Geology. For unknown geological reasons Pennsylvania is peculiar for exhibiting the Palaeozoic system in its maximum development, that is, from the Permian forma tion down to the base of Murchison s Lower Silurian, with a total thickness of more than 40,000 feet at the eastern outcrops, diminishing to half that amount in the western counties. As all the formations are thrown into great anticlinal and synclinal folds, and cut through trans versely by the rivers, they can be measured along numerous continuous and conformable section lines. Near Harris- burg, at Potts ville, and at Mauch Chunk the Carboniferous, Devonian, and Upper Silurian rocks, standing vertical, show a cross section 5 miles thick. At the Delaware and Lehigh water-gaps the Lower Silurian slates are 6000 feet thick. In Canoe valley the underlying Lower Silurian limestones have been measured 6500 feet thick. In the south-western corner of the State about 1000 feet of Permian rocks overlie the Coal-measures proper. Thus the following Palaeozoic column can be studied with peculiar advantages in Pennsylvania, many of its more important stages either becoming greatly attenuated or wholly dis appearing when followed into the neighbouring States of New York, Ohio, and Virginia. Geological Map of Pennsylvania. {Permian, or Upper Carboniferous. Upper productive Coal-measures^ Barren measures L Middle Carboniferous. Lower productive Coal-measures I 12. Pottsville conglomerate 1 1. Mauch Chunk red shale L Carboniferous . 10. I ocono grey sandstone J 9. Catskill red sandstone ; Upper Devonian. fChemung and Portage shales ; Middle Devonian, o J Tennessee, Hamilton, and Mar-"j | cell us r Lower Devonian. V. Upper Helderberg limestone J 7. Oriskany sandstone. 6. Lower Helderberg limestones ^ 5. Clinton shales [-Upper Silurian. 4. Medina and Oneida sandstones ) 3. Hudson river and Utica slates S 2. Trenton and Great Valley lime- [-Lower Silurian. stones I 1. Potsdam sandstone. The geology of south-eastern Pennsylvania is not under stood. There can be no doubt that the copper-bearing porphyritic Huronian system is well represented in the South Mountains, south of the Chambersburg fault, on the borders of Maryland ; but the systematic age of the gneisses, mica schists, garnetiferous schists, serpentine and chrome iron rocks, of the Philadelphia belt, commencing at Trenton, crossing the Schuylkill river on a section line 15 miles wide, and extending through DelaAvare and Chester counties into Maryland, is still under discussion, some geologists considering them of pre-Cambrian age and others regard ing them as metamorphosed Silurian rocks. They contain minute quantities of gold and are evidently a prolongation of the great gold-bearing belt of Virginia and the Carolinas. Minerals. The mineral resources of Pennsylvania have never been exaggerated except by those who compare its iron-mines with those of other States. It possesses a virtual monopoly of anthra cite. The output of rock-oil is still amazing. The bituminous, coking, and block coal district is only one large part of an enormous area which includes eastern Ohio, West Virginia, middle Tennes see, and northern Alabama ; and the ranges of iron-ores extend through New Jersey and New York into New England and Canada, and through Maryland, Virginia, North Carolina, ami eastern Tennessee into Alabama, with no sensible difference of quantity or quality in either direction. But Pennsylvania has the advan tage over other States of a first plant, both in iron-works and coal mines, and in a consequent multiplication and concentration of capital for these industries, which must keep cfacile princeps in this respect for a long time to come. Sooner or later she must take a second rank in iron, but never in coal and coke. It is possible that the oil-fields of the three States to the south and west of her may become as productive as her own, although no signs of such an event are visible yet to geologists ; but no contingency of events can affect her absolute control of the anthracite market.