No. 3.] SPENCER — PALAEOZOIC GEOLOGY. 141 



green indurated shales which on weathering become red. It 

 was obtained from a freshly broken surf'^ce at an artilicai ditch 

 in Ainsley's Hollow, west of Hamilton. 



Silica 50-2 



Alumina 12-0 



Iron Protoxide 1-5 



Lime 17-7 



Magnesia 5-8 



Carbon Dioxide 11-6 



98-8 

 A portion of the lime and magnesia was present as silicates, 

 some of which was decomposed by acids. In various analysis of 

 the Medina shale, made by Dr. Sterry Hunt, less than one per 

 cent, of fixed alkalis was found to be present. Under the micro- 

 scope, these rocks exhibit small crystalline dolomitic particles 

 scattered through the mass, sometimes uniformly, and sometimes 

 in thin layers. 



From the geological evidence adduced by the Ohio Geological 

 Survey (as will be noticed under the Clinton formation), the 

 Hudson River formation was raised up into a shore line be- 

 fore the deposition of the members of the Niagara group. In 

 the State of New York the Medina seas laved the shores of the 

 Shawangunk Mountains, whence the pebbles for the conglome- 

 rate of the lower portion of the series were derived. The western 

 margin of the sea was bounded by the " Cincinnati Arch," which 

 has been an upland since the close of the Carabro-Silurian Age. 

 The arenaceous material of the Medina series was obtained 

 largely from the adjacent highlands to the eastward, although 

 a portion of the sediments that form the " Gray band " was pro- 

 bably derived from the denudation of the more siliceous portions 

 of the Hudson River formation of the Canadian shores. 



The shaly beds of the Hudson River series, and particularly 

 those of the Utica formation of the Canadian highlands, formed 

 an abundant source whence denudations could derive an ample 

 supply of clay to produce the wide-spread off-shore deposit of 

 Medina shales in the northern portion of the sea. The period 

 was generally one of subsidence until its close, when the "Gray 

 band " was deposited, to be followed by the Clinton shallow 

 seas, which were to be filled up with impure limestones, alternat- 

 ing with muddy sediments brought down from the adjacent 

 shores. 



