298 C. SCHUCHERT — LOWER HELDERBERG-ORISKANY FORMATIONS 



Summary 



In pages 245-252 it was shown that the upper limit of Murchison's 

 Upper Silurian has been and still is vague, because the normal marine 

 (anna gradually succumbed to local conditions, associated with the pro- 

 duction of red sediments. In England and in the United States the 

 eurypterids are the prevailing or characterizing fossils of these land- 

 locked waters. Since the Tilestones and Downtonian of Great Britain 

 are approximately synchronous with the Cayugan group of America, it 

 is convenient to use both as the terminating strata of the Siluric system. 

 Stability in taxonomy has for its basis original definition and priority 

 in publication. This rule will not permit of the Siluric being extended 

 to include the Helderbergian, which contains a fauna having almost 

 nothing in common with this system in the typical area. 



The Helderbergian sea transgressed widely over the land areas. Earth 

 oscillations appear to he gentle throughout Lower and Middle Devonic 

 times in eastern North America, the general tendency, however, being 

 one of transgression, culminating in a continuously sinking sea bottom 

 in the northeastern Mississippian sea during Upper Devonic time. 



The Siluric system of America has three subdivisions — the Oswegan, 

 Niagaran, and Cayugan. These correspond to the English Llandovery, 

 or Valentian; Wenlock, or Salopian; and Ludlow, or Downtonian. 

 The English and American Siluric horizons have much in common, as 

 may be seen by the table on page 251. 



In pages 252-268 the Old Red Sandstone and the original Devo- 

 nian of Murchison and Sedgwick are described. The lower limits of the 

 Devonic system, and particularly a lowest Lower Devonic fauna, have 

 not been indicated by Murchison. By general consent, the stratigraphy 

 and paleontology of the Devonic system have been determined in cen- 

 tral Europe, and especially for the Lower Devonic in the Rhineland. 



Since de Verneuil's visit to America (1847), the Oriskany formation has 

 been generally accepted as the base of the Devonic in this country. It 

 has its equivalents in the Rhineland in the Siegen grauwacke, or zone of 

 Spirifer primsevus. Similar horizons appear in part in the Lower Wieder 

 Schiefer of the Hartz, in the hamlet of Erbray in the lower Loire, France; 

 at Looe, in Cornwall, England, and possibly in the Lynton slates of 

 North Devon. 



All the foregoing European localities are of accepted Lower Devonic 

 age. In Rhineland beneath the Siegen grauwacke are great masses of 

 other Lower Devonic rocks, but their faunas do not readily correlate 

 with American horizons. While part of the fauna of the Lower Wieder 



