526 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



raine in New York and to the corresponding top of the Mount Hope bed 

 of the Maysville group at Cincinnati. Above this horizon correlation of 

 Ordovician and basal Silurian deposits by means of lithologic similarity 

 is no longer possible. 



Correlation by similarity in succession of rock types is an even more 

 feasible proposition between central Kentucky and middle Tennessee 

 than between the former and New York. In this case it is practical not 

 only for the whole of the Ordovician, but also for most of the Silurian. 

 Certain formations are absent in one and present in the other, but these 

 lapses are easily detected by the paleontologist. The only really perplex- 

 ing difference is in the Eichmond deposits, and this proves to be due to 

 provincial distinctions. The close similarity in lithologic units existing 

 in Kentucky and Tennessee is shown by the following list of sixteen 

 Ordovician and Silurian formations and members clearly recognizable in 

 the two areas: Middle Stones Eiver, Lebanon limestone. Carter lime- 

 stone, Lowville limestone. Hermitage formation, Bigby limestone, Flani- 

 gan chert, Perryville limestone, Catheys formation, Fairview limestone 

 (included in the Leiper formation in Tennessee), Arnheim shale. Brass-. 

 field limestone, Osgood limestone, Laurel limestone, Waldron shale, and 

 Louisville limestone. One Ordovician formation, the Kimmswick, two 

 Eichmondian, and two or three Niagaran beds beneath the Louisville 

 limestone have been found in Tennessee, but are so far unknown in Ken- 

 tucky, while three of the Ordovician zones in Kentucky — the Curdsville, 

 Eden, and McMillan — and the Saluda- Whitewater beds of the Eichmond- 

 ian, do not outcrop in Tennessee. 



(6) Apparent lithologic units often include stratigraphic hiatuses. — 

 Lithologic units, using the term in the stratigraphic sense, may or may 

 not signify continuous deposition. Very important stratigraphic bounda- 

 ries, perhaps marking great hiatuses, are often included in an apparent 

 lithologic unit. Indeed, the contact plane may be so tightly cemented 

 that the rock above and beneath it forms a single layer. The latter is a 

 common occurrence when a fine-grained limestone, like the Lowville or 

 the Stones Eiver, is on either side of the line. In the Mohawk Valley of 

 New York the top surface of the Lowville is often smoothly planed, either 

 parallel with or slightly across the bedding planes, and riddled with verti- 

 cal worm bores an inch or two in length. Generally not a vestige of detri- 

 tal matter separates this surface from the overlying darker limestone (of 

 some later Black Eiver or early Trenton age), which fills the burrows and 

 forms so close a joint that hand specimens showing the contact are easily 

 procured. 



