walcott.] CORRELATION. 415 



tion as the bituminous schists and the Orthoceratite limestoues of Swedon and Rus- 

 sia. The Utica slate and the Hudson River group, with- the Graptolites at their 

 base, represent the Graptolite slates which in Sweden succeed to the red Orthocera- 

 tite limestone. They are also the same as those of Bain in France. 1 



Of the Upper Silurian rocks the Niagara group is considered the 

 exact equivalent of the limestones and slates of Wenlock and of Goth- 

 land. The Clinton group, with Pentamerus oblongus, represents the 

 extreme upper part of the Caradoc sandstone, or a stage intermediate 

 between the Wenlock and Oaradoc beds; while the five inferior 

 groups of the Helderberg division represent the rocks of Ludlow. The 

 correlation is then extended to the Devonian, and lists are given of 

 identical species of fossils on the two continents. 



Hall. — The essay of M. de Verneuil was followed in 1851 by a paper 

 on the " Parallelism of the Paleozoic deposit of the United States and 

 Europe," by Prof. James Hall. In discussing the criteria of correlation 

 he says : 2 



The simplest principles of elementary geology teach us that sedimentary beds, hav- 

 ing the same thickness and the same lithological characters, can not have spread 

 over an area so wide as that now included between the European and American con- 

 tinents. All sedimentary deposits must vary in character at remote points, as the 

 physical conditions of the ocean can not be presumed to have remained the same over 

 a wide extent of surface. Under such circumstances absolute parallelism is not to 

 be sought for or expected. Calcareous deposits, as would naturally be supposed, have 

 been found to be wore persistent and more uniform in the character of their fossil con. 

 tents ; but these, over some portion of their extent, have often been invaded by 

 argillaceous and arenaceous sediments, and the fauna is found to be in a greater or 

 less degree influenced by such circumstances. In distant and disconnected localities 

 we are compelled to base our opinions of the equivalency of beds upou the organic re- 

 mains which they contain ; and when we reflect that the nature of the sediment may 

 in a jjreat degree influence the character of the fossils we shall not fail to recognize 

 the necessity of keeping the character of these lithological changes in view. * * # 



Besides the changes which take place in the nature of the sediment deposited upon 

 the bed of the ocean, we are to look to other important conditions which may affect 

 the fauna, and consequently our conclusions regarding the equivalency of forma- 

 tions. Among the most important of these is the depth of the ocean in which the 

 animals lived, since we know that certain species are confined within certain zones, 

 depending on the depth of the water. 



There are other conditions which sensibly affect the distribution of organic life, 

 such as temperature, pressure, and light. # * * In investigating the fauna of 

 "past ages we see nothiug to lead us to believe that the same great laws which regu. 

 late the distribution of species did not then operate with as great effect as at the 

 present time. 



In speaking of the attempted correlation between the European and 

 American rocks Prof. Hall remarks : 



All the attempts to show that the parallelism of American and European paleozoic 

 deposits have been with a view to find a correspondence with the European stand- 

 ard where the series is confessedly incomplete, and where it has suffered during its 

 deposition, or subsequent to it, many disturbances. 3 



1 Op. cit., p. 361. 



2 Report on the Geology of the Lake Superior Land District, by J. W. Foster and J. D. WhitDey ? 

 Part II, 1851, pp. 286,287. 

 3 0p.0it,p.29i 



