326 



GEOLOGY. 



in the condition of sandstone, and the greater part of the mud in the 

 condition of shale, and most of the limestone is still essentially non- 

 metamorphic. This is true over the great interior, but where dynamic 

 action has been great, and where the original position of the strata 

 has been greatly changed, the changes in the rock have also been great. 1 

 Thus in the Taconic mountains of New York, Vermont, and Massachu- 

 setts, the limestone is mainly in the condition of marble, the sand- 

 stone and quartzite have been largely changed to quartz schist and 



Fig. 140. — Section showing the position and relations of the Ordovician beds in the 

 mountains of Arkansas. Length of section, about H miles. (Penrose, Ark. 

 Geol. Surv.) 



the shales to slate and schist. The same is true in some other parts 

 of New England, and at various points in the Appalachians south 

 of New York. 



Exposure of the Ordovician rocks. — Theoretically, the Ordovician 

 rocks, like those of other systems, may be found at the surface where 

 they were never buried, and where any cover they once possessed 

 has been worn away. The areas where they have never been buried 

 fall into two categories: First, the elevation of a land-mass about 

 which Ordovician sediments had gathered would have given the land 



1 See New York City (N. Y.), Holyoke (Mass. -Conn.), and Hawley (Mass.) folios, 

 U. S. Geol. Surv. 



