THE ORDOVICIAN PERIOD. 309 



older formations during the process of their decay, and brought to 

 the sea in solution by the same streams which brought the mud in 

 suspension. 



While the rocks of the Ordovician system were as widely distributed 

 as the Ordovician seas, their present exposures (Fig. 129) are much 

 less extensive, for they are to be seen only where they have either 

 never been covered by later formations, or where the beds which 

 once overlay them have been removed by erosion. These areas, taken 

 together, are relatively small. Elsewhere, where the Ordovician rocks 

 exist, they are buried by later deposits, and are, for the most part, 

 inaccessible. They are known, so far as known at all, only by infer- 

 ence, and by the meager data afforded by borings and excavations. 

 But even by these imperfect means, it is known with a reasonable 

 degree of certainty that the Ordovician system underlies much of 

 the eastern interior of North America, though concealed by later beds. 

 The general stratigraphic relations of the Ordovician system where 

 the strata have not been greatly deformed are shown in Fig. 136. 



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

 uously submerged during the Ordovician as during earlier periods, and 

 that in them the Ordovician strata overlie conformably those of Cam- 

 brian age. 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 ocean, 

 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. 

 New York section. — The strata of the Ordovician, like those of 

 every other system, are divisible into series and formations, both on 

 lithological and paleontological grounds. In North America the for- 

 mations were first studied systematically in New York, and the New 

 York section remains in some measure the standard to which others 

 are referred. On the basis of variations in the formations themselves 

 and in the fossils which they contain, the system in New York was 

 early divided into five formations. Numbered in the order of their 

 sequence, these subdivisions are as follows: 



5. Hudson River formation 



Ordovician svstem, 



4. Utica formation 

 3. Trenton formation 

 2. Chazy formation 

 b 1. Calciferous formation 



