330 GEOLOGY. 



far their exposure is due to the subsequent removal of beds which 

 once overlay them. It is barely possible that in the Taconic region 

 of western New England and eastern New York, Ordovician beds are 

 exposed which were never covered, except by glacial drift. Some 

 portions of the exposed Ordovician in New York, Wisconsin, and other 

 States may possibly never have been buried beneath marine deposits 

 of later age, though this can hardly be affirmed. It was formerly 

 thought that the Ordovician beds of southwestern Ohio and adjacent 

 parts of Indiana and Kentucky were arched up at the close of the 

 Ordovician to such an extent as to give rise to an island which was 

 never afterward covered by the sea; but it now appears that the final 

 emergence of the "arch" occurred at a later time. The extensive 

 exposure of the Ordovician formations in this region was therefore 

 probably brought about by the removal of the younger beds which 

 once overlay them (Fig. 145). The same is true of the Ordovician out- 

 crops in central Tennessee (Fig. 146). In the Appalachian system, 

 the exposures of the Ordovician beds are almost wholly due to the 

 removal of the strata which once overlay them. The outcrops along 

 the Ouachita uplift in Arkansas, and most of those farther west, prob- 

 ably fall into the same category. 



The Ordovician formations are wanting in some regions where 

 their presence might have been expected. Thus in the Black Hills 

 of South Dakota, where, it will be remembered, there is an area of 

 Proterozoic rock surrounded by a border of Cambrian, Ordovician 

 strata are known to be exposed at but few points. Theoretically 

 there might have been a belt of Ordovician rocks surrounding the 

 Cambrian, just as the Cambrian surrounds the core of older rock. 

 The Ordovician system as well as the Upper Cambrian probably once 

 covered the Black Hills area. Where the system is not exposed about 

 the Cambrian, the Ordovician strata were probably removed by erosion 

 during some post-Ordovician period when the region was above sea- 

 level. When the region was again submerged, new deposits prob- 

 ably covered all the Ordovician strata which had escaped destruction 

 (Fig. 147). These later beds have been removed so as to discover 

 the Ordovician at but few points. 



Thickness. — From what has been said it is easy to infer that the 

 thickness of the Ordovician system varies greatly (see p. 248). In 

 the Appalachian mountains it is to be measured by thousands of feet, 



