SILURIAN AGE. 169 



the bottom of the Upper Llandovery; the Upper Llandovery is thus made 

 Upper Silurian, and the lower, Lower Silurian. 



In Bohemia, Barrande's formations E to H are referred to the Upper Silu- 

 rian, consisting of schists, part Graptolitic, with trap. In Norway, Encrinal 

 schists, Coral and Pentamerus limestone, Lower Argillaceous schists. 



I. LOWER SILURIAN. 



3. Hudson Period (4). 



2. Hudson Epoch: Hudson Eiver shale (46). 



1. Utica Epoch : Utica shale (4 a). 



In Great Britain, Lower Caradoc sandstone, and upper part of Llandeilo 

 flags. In Bohemia, part of Barrande's formation D. In Sweden, Orthoceratite 

 limestone and Encrinal schists, flagstone. Angelin's Region D. 



2. Trenton Period (3). 



2. Trenton Epoch (3 b) : Birdseye limestone (3 6 / ), Black 

 Eiver limestone (Sb // ), Trenton limestone (3 b /// ). 



1. Chazy Epoch (3 a) : Chazy limestone. 



In Great Britain, Bala limestone and Llandeilo flags. In Bohemia, Bar- 

 rande's formation D. In Sweden, Angelin's C, Orthoceratite limestone. In 

 Russia, schists and Orthoceratite limestone, called the Pleta ; Ungulite grit of 

 Pander beneath the limestone. 



1. Potsdam, or Primordial Period (2). 



2. Calciferous Epoch (2 b) : Calciferous sandrock in New 

 York. Sandstone with very thick shales and some limestone 

 forming the larger part of the " Quebec" group of Canada and 

 the Taconic of Emmons. Magnesian limestone of the Missis- 

 sippi valley. Sandstones and Magnesian limestone in Eastern 

 Tennessee. 



1. Potsdam Sandstone (2 a): Potsdam sandstone in New 

 York, and sandstone with some limestone in the West. Thick 

 slates, sandstone, and some limestone in and along the Green 

 Mountains and the Alleghanies. Chilhowee sandstones, Ten- 

 nessee. 



In Great Britain, Lingula flags ; hard sandstones with schists below at Stiper 

 Stones in Shropshire; Black schists at Malvern; Tremadoc slates; Skiddaw 

 slate. In Bohemia, Barrande's Primordial Zone, C. In Norway and Sweden, 

 Angelin's A and B, Fucoidal sandstone and millstone, Olenus beds, schists. 



Explanation of the Geological Map. 



The annexed map of New York and a part of Canada exhibits 

 the surface-rocks of the region. As remarked under the Azoic 



