THE ORDOVICIAN PERIOD. 



327 



composed of pre-Ordovician formations a border covered by Ordo- 

 vician sediments, which might never have been submerged and buried 

 (see Fig. 141); and second, any part of the Ordovician sea-bottom 



Fig. 141. — Diagram illustrating the exposure of Ordovician beds about a center of 

 older rock. The diagram illustrates exposure of the Ordovician beds by emer- 

 gence after the Ordovician period. 



distant from preexistent land might have been raised to the estate 

 of land at the close of the Ordovician period. If never covered by 

 younger beds, it would belong in the category here specified (Fig. 142). 



Fig. 142. — Diagram illustrating the exposure of Ordovician beds in a region where 

 older formations do not appear at the surface. The exposure results from emer- 

 gence. 



The situations where the exposed Ordovician strata were once 

 covered, but subsequently laid bare by the removal of the overlying 

 beds, likewise fall into two categories corresponding in some sense 

 to those just mentioned. About the land areas of the Ordovician 

 period, the water was presumably shallow, and here a given amount of 

 lowering of the sea-surface relative to land would be more likely than 



Fig. 143. — Diagram illustrating the exposure of Ordovician beds in an area where 

 they were once concealed by younger beds, S, in a region where the younger beds 

 are grouped about a center of older rock. 



elsewhere to bring the beds covering the Ordovician into such a posi- 

 tion as to permit of their removal. This is illustrated by Fig. 143, 



