256 Nezv York State Museum 



of Pennsylvania the strongly folded and eroded Ordo- 

 vician beds are overlain in some places by Upper Silurian 

 strata, in other places by Middle or Upper Silurian or 

 even Lower Devonian beds and everywhere the uncon- 

 formity between the two systems, marking the old erosion 

 surface or plane, is well shown although later disturbance 

 has taken place. Metamorphism of the sediments in- 

 volved in this disturbance was brought about by the heat 

 generated by the folding and by the intrusion of igneous 

 material in many parts of New England and in eastern 

 Canada. The limestones of Vermont were changed into 

 marbles and the mudrocks or shales of Vermont and 

 eastern New York became roofing slates. Sandstones 

 were altered into quartzites. 



The interior sea apparently was entirely drained, since 

 there are no deposits transitional to the Silurian found 

 in that area ; but it was not long before the sea had again 

 encroached upon the Mississippi valley. Large areas, 

 however, in the west and northwest remained land for 

 long periods. The first deformation movements in the 

 middle states started in the Middle Ordovician (Trenton 

 time) in low folds which, though later submerged, were 

 arched up again by lateral compression at the close of the 

 period during the time of the Taconic Disturbance. 

 Among these were the domes of the Cincinnati anticline 

 which were not only elevated but greatly enlarged at the 

 close of the period. This fold has its longer axis in a 

 northeast-southwest direction and extends from the 

 southern border of Tennessee to southwestern Ohio. Its 

 presence and character is particularly notable at the sur- 

 face by doming of the outcropping rock formations at the 

 northern and southern ends of the anticline, the northern 



