From introduction: In December, 1892, as the result of field work on the part of Dr. G. H. Williams in the northern and of the writer in the southern portion of South Mountain, there appeared a preliminary description of two of the rock types, in which their identification as ancient volcanics was announced. In this bulletin it is proposed to substantiate that identification with more detailed proof. It is further proposed to show that these ancient igneous rocks were, at the time of their consolidation, identical in character with their recent volcanic analogues, and that their present differences are due to subsequent changes, chief among which has been devitrification. It is also proposed to recognize these facts in the nomenclature.

From introduction: In his report on the Geology of Pennsylvania,' Professor Lesley summarizes his opinion upon the geology of the South Mountains and Formation No. 1. In this, Formation No. 1 is referred to the Upper Cambrian, and a detailed description is given of its occurrence along the South Mountains and across the State to the Delaware. The superjacent limestone, No. 2, is referred to the Ordovician.' Fuller reference will be made to Professor Lesley's descriptions in speaking of the rocks of York County.

Map shows late nineteenth century Pennsylvania counties, towns, and railroads. Relief shown by hachures. Scale not given.