354 



SCIENCE 



[N. S. Vol. XXIX. No. 739 



tional, and may be classed witii either Trenton 

 or Chazy. These faunas and their distinct- 

 ness in their calcareous as well as graptolitic 

 phases, are well known. The last of these 

 faunas, the Trenton-Cincinnati fauna, is of 

 almost worldwide distribution, characterizing 

 the foreign Upper Ordovicic. 



For purposes of intercontinental correlation, 

 the graptolites are most satisfactorily em- 

 ployed. The world over, Tetragraptus and 

 Phyllograpius characterize the lowest Ordo- 

 vicic, while Didymograptus hifidus further 

 represents the highest part of this lower divi- 

 sion. In England, the Arenig rocks include 

 these graptolite zones, and on the continent of 

 Europe and in Australia, etc., this Arenigian 

 division or lower Ordovicic is marked by the 

 same graptolites. In America the shales 

 carrying these graptolites are now regarded as 

 of Beekmantown age, and the correlation of 

 Beekmantown with Arenig is generally ac- 

 cepted. Erech, indeed, draws the line some- 

 what lower, making the Beekmantown (Cal- 

 ciferous) the equivalent of the Tremadoc and 

 part of the Arenig only. Freeh, however, 

 draws the dividing line between the Beekman- 

 town and Chazy below the Eort Cassin beds, 

 so that a part of his Chazy is really Beekman- 

 town in the accepted sense. From this it 

 appears that Lower Ordovicic in America 

 should include the Beekmantown only, apply- 

 ing this term to the formation in its maximum 

 development of 2,500 feet. The term Beek- 

 mantownian would, therefore, be suitable as 

 an American equivalent of Arenigian or 

 Lower Ordovicic. The Little Falls dolomytes 

 of the Mohawk Valley represent only the 

 lower part of this series, a similar horizon 

 being probably represented by Cushing's 

 " Potsdam " and Theresa formations of north- 

 western New York. Such a classification 

 would end the Lower Ordovicic with the 

 termination of the great retreatal movement 

 of the sea, which, as shown elsewhere, resulted 

 in the almost complete emergence of the North 

 American continent. The Middle Ordovicic 

 of North America would thus begin with, and 

 be characterized throughout by, an equally 

 great transgressive movement, which, with 



minor oscillations, again submerged the con- 

 tinent nearly to the extent it had suffered 

 before the emergence. 



That the Chazy, with a maximum thickness 

 of 2,500 feet, makes an adequate representa- 

 tion of the Middle Ordovicic can hardly be 

 questioned. Its fauna is distinct, though not 

 without its relationships to both preceding and 

 succeeding faunas. In fact, the Upper Stones 

 River phase of the fauna is so closely related 

 to the Black River fauna that the horizon of 

 the latter is regarded by some as more prob- 

 ably belonging to the Chazy. The Stones 

 River and Lowville faunas seem to be facies- 

 faunas of the Chazy, rather than definite in- 

 dices of the subdivisions of this faima. Thus- 

 while in general the Stones River seems to be- 

 a late phase of the Chazy, succeeding the 

 Ohamplain phase of the Chazy, throughout the 

 interior, the order seems to be reversed in the- 

 South Mountain area of Pennsylvania and'^ 

 adjoining districts, where Stose found the- 

 Upper Shenandoah to contain a Stones River- 

 fauna followed by a Ohazy-Black River fauna 

 (Chambersburg limestone). Unless there is 

 an error here, the Stones River fauna and* 

 typical Chazy fauna (i. e., that of the St. 

 Lawrence embayment) must be considered as 

 contemporaneous. From what has already 

 been said of the relationships of Trenton and' 

 Utica deposition, the former must be classed 

 with the Upper Ordovicic. This is further 

 shown by the faunal unity of these formations) 

 many of the most characteristic brachiopods, 

 bryozoa, mollusks and trilobites making their 

 first appearance in the Trenton and continuing 

 through the Cincinnati beds. These types, as 

 already noted, are of wide distribution and 

 are characteristic only of the Upper Ordovicic 

 or Caradocian of Europe and elsewhere, and 

 more especially of the higher zones of this 

 division. 



The dividing line between Middle and Upper 

 Ordovicic is drawn in Europe above the zone 

 of Ccenograptus gracilis. This graptolite 

 characterizes the Normanskill zone of Amer- 

 ica, which lies below the Trenton limestone 

 in the Hudson Valley and probably represents- 

 Black River horizon if not late Chazy, Ac- 



