118 Barrande, Logan and Hall 



rocks, and in the upper part of the Hudson River group at the 

 west. Dictyonema is a genus known from Lower Silurian to 

 Devonian strata. 



Graptolithus proper extends to the Clinton group of New York ; 

 and the same is true of Reteograptus. Tamnograptus occurs in 

 the rocks of the Hudson River group near Albany, and in the 

 Quebec rocks. Phylhgraptus and Retiolites are known in the 

 Quebec rocks only ; while the typical form of Dendrograptus oc- 

 curs in the Potsdam sandstone, and, likewise, in three other spe- 

 cies, in the Quebec rocks. 



We find, therefore, in the other genera, except trilobites, very 

 little satisfactory evidence on which to rely in the present state 

 of our knowledge, for determining the position of these strata. 



In the present discussion, it appears to me necessary to go fur- 

 ther, and to inquire in what manner we have obtained our present 

 ideas of a primordial, or of any successive faunse. I hold that in 

 the study of the fossils themselves there were no means of such 

 determination prior to the knowledge of the stratigraphical rela- 

 tions of the rocks in which the remains are inclosed. There can 

 be no scientific or systematic palaeontology without a stratigra- 

 phical basis. Wisely then, and independently of theories, or of 

 observations and conclusions elsewhere, geologists in this country 

 had gone on with their investigations of structural geology. The 

 grand system of the Professors W. B. and H. D. Rogers had been 

 wrought out not only for Pennsylvania and Virginia but for the 

 whole Appalachian chain ; and the results were shown in nume- 

 ous carefully worked sections. In 1843, '44 and '45 I had myself 

 several times crossed from the Hudson River to the Green Moun- 

 tains, and found little of importance to conflict with the views ex- 

 pressed by the Professors Rogers in regard to the chain farther 

 south, except in reference to the sandstone of Burlington, and one 

 or two other points, which I then regarded as of minor impor- 

 tance. 



Sir William Logan had been working in the investigations of 

 the geology of Canada ; and better work in physical geology has 

 never been done in any country. 



This then was the condition of American geology, and inves- 

 tigators concurred, with little exception, in the sequence based 

 on physical investigations. As I have before said, our earliest 

 determinations of the successive faunae depend upon the previ- 

 ous stratigraphical determinations. This I think is acknowledged 



