Morton, John S. A History of the Origin of the Apellation Keystone
State as Applied to the Commonwealth of Pennsylvania: Together with Extracts from Many
Authorities Relative to the Adoption of the Declaration of Independence by the Continental
Congress, July 4th, 1776; To Which is Appended the New Constitution of Pennsylvania with an
Alphabetical Contents. Philadelphia: Claxton, Remsen & Haffelfinger, 1874.