82 HISTORICAL GEOLOGY 



of quite different character from those of the Cambrian. Clastic 

 sediments, such as conglomerates, sandstones, and shales, were the 

 dominant Cambrian sediments, while, throughout the Lower and 

 Middle Ordovician, limestones greatly predominate (Fig. 44). 

 Mid-Ordovician is generally regarded as one of the greatest lime- 

 stone making times in the earth's history, though it should not be 

 inferred that limestones were then universally made in the seas, 

 because those areas of deposition close to, or receiving wash from, 

 the lands show clastic sediments. Middle Ordovician, especially 

 Trenton, limestones are remarkably widespread, occurring in New 

 York, New England, New Brunswick, southeastern Canada, and 

 near Hudson Bay, across the northern part of the Mississippi 

 Basin, Black Hills, Wasatch and Uinta Mountains, and even in 

 the Great Basin. An illustration of an exception to universal 

 limestone-making during Trenton time is in the Mohawk Valley 

 region of New York, where the limestone passing eastward be- 

 comes almost wholly replaced by hundreds of feet of shale. Also 

 through the Appalachians rocks of this age contain much clastic 

 material. 



Upper Ordovician Rocks. — In the Upper Ordovician of eastern 

 North America shales and alternating shales and fine-grained sand- 

 stones (e.g. Utica and Lorraine) greatly predominate, doubtless 

 due to rejuvenation and more active erosion of the lands probably 

 accompanied by some shoaling of the water (Fig. 44) . In the west- 

 ern part of the continent limestones appear to predominate, even 

 in the Upper Ordovician. 



Thickness and Metamorphism of the Ordovician. — The 

 aggregate thickness of Ordovician strata in New York is from 2000 

 to 3000 feet; in Tennessee about 4000 feet; and in the central 

 Mississippi Valley (e.g. Missouri) only a few hundred feet. 



Among the changes which the strata have undergone since their 

 deposition we have mentioned their highly folded condition in 

 certain regions, but in New England, parts of the Piedmont 

 Plateau, and parts of the western United States the rocks are also 

 highly metamorphosed. 



Igneous Rocks. — There is no certain evidence for volcanic or 

 other igneous activity during the North American Ordovician, 

 though some granite intrusions in the Wichita Mountains of 

 Oklahoma and some very small dikes in New York may be of 

 this age. 



