MARYLAND GEOLOGICAL SURVEY 171 



sandstone and the Bed and White Medina formations as the earliest major 

 division of the Silurian. The reason for regarding the Oswego sandstone 

 as Ordovician in age has been given on a preceding page. 



In the standard Ordovician-Silurian section of New York the last 

 fauna of Ordovician age is found in the Pulaski shale where species 

 identical with Lower Maysville fossils of the Cincinnatian section occur. 

 Above the Pulaski shale are unfossiliferous gray sandstones (Oswego 

 sandstone) which in turn are succeeded by the Lower ("Red") Medina 

 forming the base of the Silurian, according to the New York geologists. 

 This Lower Medina is unfossiliferous, but in the overlying Upper 

 ("White ") Medina a fauna of Silurian types is preserved. 



In Pennsylvania and Maryland practically the same section is de- 

 veloped. The sandstones at the top of the Martinsburg shale contain the 

 Pulaski shale representative with the Lower Maysville fauna and above 

 this a gray unfossiliferous sandstone very similar to the Oswego sand- 

 stone occurs, followed in turn by the typical Eed Medina here termed the 

 Juniata formation, and by the White Medina or Tuscarora sandstone. 



In the extremely fossiliferous Upper Ordovician, Cincinnatian rocks 

 of the Ohio Valley, the equivalent of the Pulaski shale of New York is 

 included in the Lower Maysville, Fairview formation. This is succeeded 

 by the fossiliferous Upper Maysville (McMillan) formation, which in turn 

 is followed by the equally fossiliferous Richmond group. Lately it has 

 been proved by actual tracing that the Richmond group passes laterally 

 into the Lower (Red) Medina of New York. Until the recent researches 

 of Ulrich upon the paleontology and stratigraphic distribution of the 

 Richmond and related formations, the Upper Ordovician age of the Rich- 

 mond fauna had been taken for granted. In his paper on the Ordovician- 

 Silurian boundary, published in the Proceedings of the Twelfth Session 

 of the International Geological Congress, Ulrich has reviewed the fauna! 

 and physical aspects of the data relating to the age of the Richmond group 

 and concludes that the choice of the Medina including the Queenston- 

 Richmond formations by the New York geologists as the base of the 



