Author
Geological Survey of Pennsylvania
3
Pennsylvania. Department of Conservation and Natural Resources
3
Cate, Addison S.
2
Dodge, Clifford H.
2
Hartranft, Rufus C.
1
Heilprin, Angelo, 1853-1907
1
Jones, Thomas H. 1912-
1
Pennsylvania Geological survey (2d, 1874-1890)
1
Pennsylvania. Bureau of Topographic and Geologic Survey
1
Shaulis, James R.
1