Handbook of Paleontology 301 



rocks comprised in Murchison's Upper Silurian, an ar- 

 rangement now generally adopted by British and 

 American geologists. It is of interest to note that the 

 old New York term, Ontario division, as defined 

 1842-43, comprised almost exactly the same range of 

 formations now referred to the Silurian system. 



Geology. North America has as full a representation 

 of the rocks of the Silurian system as occurs in Great 

 Britain, and, as with the Ordovican and Devonian, the 

 classification used in New York is the standard for 

 America. 



At the end of the Ordovician there was a with- 

 drawal of the sea at the time of the mainly continental 

 deposition of the Oswego sandstone. Then came a re- 

 turn of the sea (Richmond or Medinan) with a flood- 

 ing from the Arctic ocean, as well as from the Middle 

 and South Atlantic and Pacific. It was during this 

 flood, when a large portion of the continent was sub- 

 merged, that a river delta of red sandstones was de- 

 posited in the Appalachian trough and extended from 

 near the southern extremity of Virginia through Mary- 

 land and Pennsylvania into New York where it appears 

 as the red sandy shales of the Queenston formation. 

 It is the erosion interval below the Richmond 

 (Medinan) series, together with the Silurian types 

 found in the fauna, that has led to the inclusion of the 

 Richmond beds in the Lower Silurian (Ulrich). Some 

 authors include all formations up to the Clinton beds 

 (lower Middle Silurian) in the Ordovician, others 

 make the division at the top of the Richmond. The 

 break at the base of the Richmond is very marked over 

 the greater part of the interior from the upper Missis- 

 sippi valley west to the Rocky mountains and north 



