328 



GEOLOGY. 



which represents the Silurian strata, S, as once covering the Ordo- 

 vician, 0; but the removal of the former has exposed the latter. Again, 

 if any part of the sea-bottom not immediately associated with a pre- 

 existing land area were lifted above its surroundings, not at the close 

 of the Ordovician, but at some later time, the uppermost strata would 

 be in a position favorable for removal. With their removal, the Ordo- 

 vician beds beneath would be discovered (see Fig. 144). In the cases 

 illustrated by Figs. 141 and 143, the Ordovician outcrops would appear 

 around or against the outcrops of older formations ; in the cases repre- 



Fig. 144. — Diagram illustrating the exposure of Ordovician beds where they have 

 been discovered by erosion in a region where older formations do not appear at 

 the surface. 



sen ted by Figs. 142 and 144, the Ordovician system would itself be the 

 center about which younger beds outcrop. 



The map (Fig. 129) shows the principal areas where the Ordovician 

 strata appear at the surface, and the relations of their outcrops to 

 pre-Ordovician lands (compare Figs. 38 and 95). From this map it is 

 seen that outcrops of Ordovician are found along the western border 

 of Appalachia; in Wisconsin, South Dakota, Missouri, Texas, and several 

 States farther west, and also at many points north of the United 

 States. In most of these situations the outcrops of the Ordovician 

 are adjacent to exposures of Cambrian or pre-Cambrian formations. 

 Where the outcrops of the Ordovician strata are next a pre-Cam- 

 brian formation, rather than next the Cambrian, it means that the 

 margin of the Ordovician sea extended landward farther than the 

 Cambrian for that locality, and the deposits of the later period lapped 

 over and buried those of the earlier. In the vicinity of Cincinnati, 

 in Ohio, and adjacent States, the Ordovician strata appear at the 

 surface in an area where older formations are not seen. The rela- 

 tions are shown by Fig. 145. 



If the attempt be made to refer the individual outcrops of the 

 Ordovician strata to one of the categories already mentioned, diffi- 

 culty is at once encountered, for it is not always possible to say how 



