88 HISTORICAL GEOLOGY 



How do we know that the Taconic disturbance took place 

 toward the close of the Ordovician period? Strata of the next 

 succeeding period (Silurian) rest directly in places upon the eroded 

 edges of late Ordovician rocks; hence it is obvious that the disturb- 

 ance occurred before the Silurian strata were deposited. Also the 

 disturbance doubtless began before the close of the Ordovician 

 period. This is borne out by the fact that, for example, in central 

 New York a distinct eroded surface at the summit of the Frankfort 

 shales proves that region to have been dry land before the end of 

 the period, this uplift quite certainly having been produced by the 

 early movements of the Taconic disturbance. 



Another feature which must not be overlooked is the profound 

 metamorphism of the strata along the main axis of the Taconic 

 Range. The very intense compression, accompanied by heat and 

 moisture, caused the deeply buried strata, along the main axis of 

 the uplift, to become rather plastic, and hence the sediments be- 

 came more or less foliated and crystallized into the various meta- 

 morphic rock types, the limestone becoming marble, the shale 

 becoming slate or schist, and the sandstone becoming quartzite. 



In Xew Brunswick, Silurian strata rest upon the eroded edges 

 of upturned Ordovician strata, and this upturning may have been 

 coincident with the Taconic disturbance. 



Sufficient lateral pressure was brought to bear in a portion of 

 the Mississippi Basin, during the latter part of the period, to 

 produce a long, very low arch in the rocks from southern Ohio into 

 Tennessee. This has been called the " Cincinnati Anticline. " 



The late Cambrian and Ordovician connection of the interior 

 sea with the Atlantic Ocean through the St. Lawrence Valley was 

 closed bv the disturbances toward the end of the Ordovician. 



Foreign Ordovician 



Map Fig. 47 gives a general idea of the relations of land and 

 sea in Europe during the Ordovician. Also, barring certain areas 

 from which the strata have been removed by erosion, the dotted 

 (shaded) portion represents the present extent (surface and con- 

 cealed) of Ordovician strata. There were two distinct provinces, 

 a northern and a southern, as proved by important differences 

 between the fossils of northern and southern Europe. Ordovician 

 fossils of northern Europe are closely related to those of Xorth 



