16 DEFINITIONS AND LAWS OF GEOLOGY. 



ward rocks of the same age on the Mississippi were called the "Lower Magnesian 

 Limestone," and geologists of that locality persist in the use of the latter name, be- 

 cause they say the word Calciferous is not admissible from the lithological character 

 of the rock. It is to be regretted that the name Calciferous has come down to this 

 generation as the name of a Group of rocks, but it is as well established as the name 

 of any other Group, and like the word Tertiary, which has no application to the 

 rocks to which it is applied, is fastened upon the science, and so interlocked with it 

 that it can not be eradicated even were it desirable so to do. With how much less 

 reason should we encourage the use of another mineralogical name, having more 

 limited application, in its stead ! 



£ 27. The rule is, the law of priority should be rigorously enforced where a 

 Group has been named, and the fossils have been so described and illustrated that it 

 may. be identified by a palaeontologist elsewhere than at the typical locality. Syno- 

 nymy is always the result of ignorance, and much of it has come from those whose 

 work has been absolutely worthless. 



§ 28. Experience has shown the impracticability of making lesser subdivisions 

 for the purpose of geological nomenclature, than Groups, especially in the present 

 state of the science, though it is eminently fit and proper to speak of the marl-beds 

 or sandstone layers in any Group, or of the Glyptocrinus or Chillis beds at any locality. 

 Minute and careful definition and description of the characters of each aud every 

 part of a Group is one thing, and the suggestion of a geological subdivision, founded 

 upon a marked peculiarity at one locality, which can not be distinguished at another, 

 is quite a different thing. It must not be supposed none of the Groups will be sub- 

 divided, but proposing a name is not establishing a Group. The Coal Measures 

 ought to be divided into Groups because of the great thickness of the fossiliferous 

 rocks, and a temporary division in some localities is indicated by the use of the words 

 Upper, Middle, and Lower Coal Measures, but great palaeontological information 

 must be acquired before any practicable subdivision can be made. 



§ 29. The stratigraphical division of the rocks of North America into Groups 

 bearing geographical names, with an approximate thickness in ascending order, is 

 as follows : 



Laurentian System, not divided into Groups 40,000 feet. 



6 . f Lower Taconic, not divided into Groups, 25,000 ' 



{J •£ \ Georgia Group, including St. John Group, 10,000 " 



H c ( Swanton Group, ? . . . 10,000 " 



f Potsdam Group, 4,000 " 



Calciferous Group, 2,000 " 



Quebec Group (very doubtful), 6,000. feet ? 



Ohazy Group, '. . . 1,000 " 



Black RiverGroup, 150 " 



Trenton Group, 1,100 " 



Utica Slate Group, 600 " 



[Hudson River Group, 1,200 " 



u C 

 u tt 



> °u 



O 3 



O. 3 

 CO 



Medina Group 2,500 " 



Clinton Group, 500 " 



Niagara Group, 800 " 



Guelph Group, | l2QQ M 



Onondaga Group, J ' 



.Lower Helderberg Group, 2,000 " 



Carried forward, 102,050 feet. 



