THE ORDOVICIAN PERIOD 



509 



entered into the new system, derived from all preceding formations 

 so situated as to be exposed to erosion, was brought from the land 

 by streams, worn from its shores by waves, or blown to the sea by 

 winds; and where terrigenous sediments failed, or where they were 

 relatively unimportant, the secretions of the animals and plants 

 accumulated, giving rise to sedimentary rocks of organic origin. 

 Even these had their ultimate source in the older formations, for 

 the mineral matter extracted from the sea to make the shells had 

 been dissolved from older formations during the process of their 

 decay, and brought to the sea in solution, often by the same streams 

 which brought the mud in suspension. 



It is probable that the larger part of the ocean basins was con- 

 tinuously submerged during the Ordovician, as during earlier 

 periods, and that in them the Ordovician strata overlie those of 

 Cambrian age conformably. Though nothing can be known 

 directly of the Ordovician system beneath the sea, it is important, 

 in the conception of the system as a whole, to remember that it 

 probably underlies most of the oceans as well as many of the 

 younger formations of the land, and that its exposed margin is but 

 a trivial fraction of its total area. 



Sections of the Ordovician 



The New York section. The Ordovician system of North 

 America was first studied carefully in New York, and the section of 

 that State is, in some measure, the standard to which others are 

 referred. The svstem in New York is now divided as follows: 



Ordovician < 



Upper Ordovician 

 (or Cincinnatian) 



Middle Ordovician 

 (or Mohawkian) 



Lower Ordovician 

 (or Canadian) 



( Richmond beds * (in Ohio and Indiana) 



j Lorraine beds 



( Utica shales 



( Trenton limestone 



j Black River limestone 



( Lowville limestone 



( Chazy limestone 



I Beekmantown limestone (calciferous) 



1 Question has recently been raised as to the propriety of including the 

 Richmond beds in the Ordovician. It has been suggested that they are per- 

 haps the equivalent of the Medina, and if so they belong with the succeeding 

 system. Hartnagle, N. Y. State Mus. Bull. 107, 1907. In Illinois, beds of 

 Richmond age are unconformable on the underlying Ordovician. Weller, 

 Jour, of GeoL, Vol. XV, p. 519; and Savage, Am. Jour. Geol., Vol. 125, p. 



