Precambrian rocks are exposed in Pennsylvania in three belts—the South Mountain belt, which is a northward extension of the Catoctin belt of Maryland and terminates a few miles southwest of Harrisburg, owing to covering by Paleozoic and Mesozoic sedimentary rocks; the Piedmont belt, which extends from the Maryland line at the Susquehanna River through the northern part of the city of Philadelphia to the Delaware River at Trenton, New Jersey, where it is likewise covered by later sediments ; and the Highland belt, which emerges from beneath the sedimentary cover 40 miles east of Harrisburg, and at Easton crosses the Delaware River into New Jersey, where it broadens to form the Highlands. The last belt, to which attention is especially directed in this paper, is about 60 miles long and attains a maximum width of 20 miles in this State.

Henry D. Rogers, first State Geologist of Pennsylvania, regarded . . .

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