256 PENNSYLVANIA in Pennsylvania, but it floats great quantities of timber. Canals along its banks convey coal and produce in great quantities. The Susque- hanna has two great branches, the North branch rising in New York, and having an irregular course of about 250 m. to Northumberland, the point of junction, and the West branch rising W. of the Alleghanies, through which it breaks eastward, 200 m. long. Below Northumber- land, 150 m. from the sea, the course of this river is more direct. The Ohio river and its branches drain the W. part of the state ; the Alleghany river branch drains the N. W. part,
- and has a length within the state of about
250 m., running mainly S. W. and S. E. ; the Monongahela branch, rising in Virginia, has a course northward within the state of 80 m. to Pittsburgh. Both these last are navigable for steamboats about 60 m. each, the latter being converted into slackwater pools. The Ohio, below their point of junction, is a great thoroughfare for steam navigation. The Ju- niata, a tributary of the Susquehanna from the west, and the Lehigh and Schuylkill, trib- utaries of the Delaware, are the principal re- maining rivers, each having canals and lock navigation. There is no considerable lake within the state, but it borders on Lake Erie for a distance of 45 m., affording access to its navigation and a superior harbor at Erie. The geological formations of Pennsylvania are limited to three of the principal divisions of the rocks. These are : 1, the azoic and eozoic formations in the southeast ; across which lies, 2, the mesozoic (new red) in a belt from 20 to 30 m. wide, extending into New Jersey and into Maryland ; 3, the palaeozoic series, from the Potsdam sandstone to the coal measures, occupying the rest of the state. The tertiary and upper secondary, developed on the E. side of the Delaware, do not extend to the other side of the river. The northern drift formation of sand and gravel, which overspreads all the states to the north, covers the N. and N. V. tier of counties, and is represented by a thin sheet of gravel, which dwindles away within 30 or 40 m. of the New York state line, except where it is traced down the valley of the Dela- ware at the east and the branches of the Ohio at the west. Along the middle portion of the N. boundary of the state the height of the ta- ble land appears to have been sufficient to pre- vent its deposit, for its bowlders and gravel are rarely detected in this portion of the state ; but the valley beds and even hill tops of the N. "W. counties are heavily loaded with drift. The gneissic rocks are limited to the S. E. counties, the gneiss occupying a margin of va- rying width along the Delaware below Tren- ton, at Philadelphia reaching up the Schuylkill about 10 m., and giving place on the northwest to a narrow belt of partially metamorphosed lower Silurian limestones, which separates it from the red sandstone. This contains the quarries of white marble that have supplied Philadelphia and the towns around. N. and N. W. of it gneiss overspreads the N. part of Chester co., and Laurentian gneiss is supposed to form the body of the Easton and Reading hills, and the South mountains west of Harris- burg. Near Phcenixville, in the new red, are the mines of lead and copper. (See LEAD.) On the range of the gneiss toward the south- west are the nickel mines in Lancaster co. Along the line of the gneiss and sandstone W. of Phoenix ville are the Warwick and other mines of magnetic iron ore, and further N. the great Cornwall mine near Lebanon, and others around Reading. South from Phila- delphia the gneiss continues round the border of the state, the edge of this formation N. of the Maryland state line coming to a point S. of Gettysburg in Adams co. Across this gneiss country, especially near the Octorara creek, run tracts of serpentine rocks, forming what are called the " serpentine barrens." In these rocks beds of chrome iron ore have been worked to a considerable extent, and at times with great profit, affording large quantities of the ore for the manufacture of chrome paints at Baltimore and for the English market. Trap dikes are of frequent occurrence, not only in the gneiss region, but especially in the belt of new red rocks overlying the older for- mations. The lower Silurian formations con- tain great deposits of hematite iron ore, as the Chestnut hill mines near Columbia in Lancas- ter co., and the numerous beds in Berks and Lehigh cos. which form the chief dependence of the blast furnaces on the Schuylkill and the Lehigh rivers; and the same lower Silurian limestones hold the same ores in Kishacoquil- las, Brush, Nittany, Sinking, Spruce creek, and Canoe valleys, and Morrison's and McConnells- burg coves in central Pennsylvania. (See AP- PALACHIAN MOUNTAINS, and IEON.) The north- ern edge of the new red ranges with the Mus- conetcong creek in New Jersey, crosses the Delaware river below Durham, and extends W. across the Schuylkill 2 m. below Reading, and the Susquehanna 5 m. below Harrisburg. It then inclines more to the south and crosses the S. line of the state near the S. W. corner of Adams co., keeping always at the foot of the South mountain or Blue Ridge. The S. edge of the same belt enters the state opposite Trenton and pursues a general W. course, pass- ing the Schuylkill 2 m. below Norristown, the Susquehanna in the W. corner of Lancaster co., and the state line in Adams co. near the S. E. corner. The tract thus included is occu- pied almost exclusively by the red sandstones, red shales, and conglomerates of this forma- tion, and by the numerous dikes of trap rock, many of which are large and are traced for miles in different directions. It is remarkable that the dip of the sedimentary rocks is not disturbed by these dikes from the uniform in- clination of the strata at angles varying from 5 to 20 toward the north and northwest. One of these dikes is remarkable not only for its straight course and extreme narrowness (sometimes
