570 . HISTORICAL GEOLOGY. 



facies of the Coblenzian fauna of the Rhine, and paralleled them with the Bohemian faunas 

 of F, G, H, taken in their entirety ; in 1880 he regarded them as a lower (not lowest) Devo- 

 nian fauna and still as a calcareous facies of the Coblenzian ; but in 1884 he appears to have 

 resumed his original position as to the age of the Hercynian, and modified his conception 

 of the parallelism with the Bohemian fauna by removing from his equivalent the lower 

 portion of F. . . . Barrois, in 1889, made the Hercynian lowest Devonian, but dif- 

 fered from Kayser (1878, 1880), in regarding it, not as a calcareous facies of the Spirifer- 

 sandstein or Coblenzian, but as such a facies of the older Gedinnian, considering the 

 Bohemian G as its equivalent." 



Kayser concluded further that the Lower Helderberg formation of America was 

 Hercynian, that is, lowest Devonian, contrary to the views of Barrande, who had made 

 it Upper Silurian, and the equivalent of the three divisions in his Bohemian system just 

 mentioned. In his recent Lehrbuch (1891), Kayser leaves the Water-lime in the Upper 

 Silurian. 



Some of the alleged Devonian characteristics of the Lower Helderberg are : its many 

 species of Dalmanites of the D. Hausmanni type ; its species of Phacops, of the type of 

 P. fecunda, and of Homalonotus, of the type of H. Vanuxemi ; the occurrence of many 

 species of Platyceras ; the special Devonian features among several genera of Brachiopods 

 and Lamellibranchs. On the contrary the formation is unlike the Hercynian in containing 

 no Goniatites, and like the Silurian in including several species of Cystideans. Mr. Clarke 

 presents in his paper a full account of the discussion ; and while he unhesitatingly refers 

 the Oriskany formation to the Devonian, on the ground of its fauna, he leaves the question 

 as to the Lower Helderberg without a decision. No attempt is made to compare the 

 American fauna with that of the Ludlow beds of England, which is really the typical 

 fauna of the later part of the Upper Silurian — the limits of the Devonian and Silurian 

 having been first laid down by Murchison and Sedgwick. 



GENERAL OBSERVATIONS ON THE UPPER SILURIAN. 

 GEOLOGICAL AND GEOGRAPHICAL CHANGES DURING THE UPPER SILURIAN. 



NORTH AMERICAN. 



In the region of the Appalachian geosyncUne. — As in the Lower Silurian, 

 the successive formations have their greatest thickness along the Appalach- 

 ian geosyncline, and at the same time limestones were the prevailing rocks 

 of the continental interior. 



The thickness of the argillaceous beds and sandstones of the East indicate 

 that during the Niagara period the deepening of the geosyncline amounted, 

 in Pennsylvania, to at least 1500 feet in the Medina epoch, over 2000 in the 

 Clinton, 1500 in the [N'iagara and Onondaga, and 500 in the Lower Helder- 

 berg, — in all 5500 feet. In the Onondaga period, the subsiding area extended 

 up into New York, west of its center; for it was there that the Onondaga 

 beds were formed to a thickness of 1000 feet, with evidence in many parts 

 of shallow-water origin. In the Lower Helderberg, and in the following 

 Oriskany periods, the greatest thickness of the beds was in the eastern half 

 of the state. 



N^o sediments for rock-making over the continent from the Atlantic Ocean. — 

 Although the Champlain channel between the St. Lawrence seas and those 

 of New York was again opened wide during the Lower Helderberg period, it 



