Upper Silurian 



390 NOTICES OF NEW BOOKS. 



in the Proceedings of the Society, as published in the various 

 numbers of this Journal. 



The older Palaeozoic or Silurian rocks of America appear to be, 

 as in England, divisible into two series — the upper and lower, and 

 Mr. Lyell has admitted in his coloured map seven sub-divisions, 

 which are thus named : — 



7. Hamilton group. 



6. Helderberg series. 



5. Onondaga salt group. 



4. Niagara and Clinton group. 



f 3. Hudson river, Utica &c. group. 

 Lower Silurian -J 2. Trenton, &c. limestone group. 

 l_ 1. Potsdam sandstone, &c. 



The Taconic system, named by Dr. Emmons from a chain of 

 mountains which form a continuation of the green mountains of 

 Vermont, and supposed to represent a group of formations more 

 ancient than the Silurian, are not considered by Mr. Lyell as 

 deserving an independent place among the rocks of the palaeozoic 

 series (vol. i. p. 246.). 



(1) The Potsdam sandstones are chiefly developed in a narrow 

 band on the south-eastern range of the great chain, extending 

 south-west from the Vermont range and along the whole of that 

 line ; and they immediately succeed and overlie the granitic and 

 gneissose rocks of Virginia, North Carolina, and Georgia. They 

 are also found occupying a broader space on the banks of the 

 great river St. Lawrence, between the north-eastern extremity of 

 Lake Ontario and the city of Montreal; and here these most 

 ancient of the fossiliferous rocks are loaded with the remains of 

 Lingula, and a small placunoid shell nearly allied to a fossil also 

 occurring in company with the Lingula in the lowest English 

 silurian beds at Builth in Brecknockshire (ii. 157.) Beds of the 

 same Lower Silurian date have also been traced on the banks of 

 the Wisconsin river, a tributary of the Mississipi, and Captain 

 Bayfield is inclined to consider a band of sandstone on the 

 southern coast of Lake Superior as the equivalent of these oldest 

 fossiliferous sandstones of Potsdam. 



(2) The Trenton limestone is much more widely distributed in 

 North America than the inferior beds of sandstone; but it appears 

 difficult to separate it in some cases from the beds of the over- 

 lying Hudson river group (3)* (ii. 49). The "blue limestone," 

 as it is called, forming the hills and table lands around Cincinnati 

 and elsewhere in Ohio, belongs to the upper part of the lower 

 Silurian group, and abounds in organic remains, consisting chiefly 

 of trilobites, brachiopodous shells, crinoidea, and many corals, the 

 latter differing considerably in specific character from those in the 



* The Hudson river group of Mr. Lyell includes a number of sandy and 

 argillaceous slates containing Lower Silurian fossils, and separating in some 

 cases the Trenton from the Niagara (Upper Silurian) limestones. 



