78 HISTORICAL GEOLOGY 



f Pulaski f 

 ~ Lp, , , ' , < Lorraine shale and sandstone. 



Cincinnatian series \ Frankfort, I 



(Upper Ordovician) [ Utica shale. 



Mohawkian series / Trenton limestone and shale. 

 (Middle Ordovician) \ Black River Kmestone. 



f Chazy limestone. 

 Canadian series J Pamelia limestone. 



(Lower Ordovician) ] Beekmantown hmestone. 

 [ Tribes Hill limestone. 



The reader should not be led to think that these New York 

 formation or stage names are the only ones now used in North 

 America. Many other, more or less local, names have been applied 

 either to formations (stages) found elsewhere but missing in New 

 York, or to formations which have not yet been definitely cor- 

 related with those of New York. It is generally agreed, on the basis 

 of priority, that when two widely separated formations become 

 definitely correlated, the name given the formation where first 

 studied is to be applied to both. In this way many of the New 

 York names have come to be used over wider and wider areas. 

 Also the kind of rock (lithologic character) making up a formation 

 in New York may or may not be the same in other areas. Thus a 

 sandstone or shale in New York may be replaced by a shale or 

 hmestone elsewhere, etc. 



In New York, and usually elsewhere, the Ordovician strata, 

 especially the Middle and Upper, and more especially the Trenton 

 beds, are wonderfully rich in organic remains, and much attention 

 has been given to the description of the fossils and the correlation 

 of the strata. Unconformities within the Ordovician system are 

 relatively uncommon, those which do occur seldom ever being of 

 more than local extent. In other words, with certain slight ex- 

 ceptions, deposition of Ordovician strata in North America was a 

 continuous process, and the subdivisions are very largely on the 

 basis of fossils. 



Distribution and Character of the Rocks 



General Distribution. — The accompanying map (Fig. 41) 

 shows the distribution of chiefly Middle and Upper Ordovician 

 rocks in North America. Some Lower Ordovician rocks are in- 

 cluded with the Cambrian on the preceding map (Fig. 26), but 



