The Collaborative International Dictionary of English v.0.48:

Niagara period \Ni*ag"a*ra pe"ri*od\ (Geol.)
   A subdivision or the American Upper Silurian system,
   embracing the Medina, Clinton, and Niagara epoch. The rocks
   of the Niagara epoch, mostly limestones, are extensively
   distributed, and at Niagara Falls consist of about eighty
   feet of shale supporting a greater thickness of limestone,
   which is gradually undermined by the removal of the shale.
   See Chart of Geology.
   [1913 Webster]