GRAPTOLITES OF NEW YORK. TART 2 59 



With one exception, all these graptolites are dendroid forms which 

 lived under the same conditions as the associated benthonic forms and have 

 therefore the same correlation value as these. Their occurrence is but local 

 and they are for this reason not apt to gain importance as zone markers. 



The solitary species of Monograptus exhibits such extreme reduction 

 in size and development that one cannot help but doubt its graptolitic 

 nature altogether on seeing it, but may well infer in view of its stratigraphic 

 position that it is a last gerontic form. 



Finally the Carboniferous rocks of Missouri have in Dictyonema 

 b 1 a i r i Gurley also given evidence of the persistence of that extremely 

 lonef-lived " genus " into Carboniferous time. 



7 Note on the continuity of the formation of graptolitiferous beds in the same 

 area and its bearing on certain paleogeographic problems 



We have seen passing before our mental vision in the preceding 

 chapter and the corresponding one of part i of the Graptolites of New 

 York a continuous procession of graptolite zones beginning with the closing 

 stage of the Cambric and continuing, apparently without interruption, to 

 the closing stages of the Champlainic. This fact of the immensely long 

 persistence of the same marine facies in one area is certainly a remarkable 

 phenomenon which becomes the more marvelous when it is taken into 

 account that this area is relatively small, all the graptolite zones being 

 restricted to the slate belt, or the Levis channel, with the exception of the 

 last two which pass beyond its western boundary, at the same time losing 

 their character as typical graptolite shales ; and further when it is considered 

 what a variety of rocks were in the same interval deposited in the epicon- 

 tinental American sea, and even in the adjoining and parallel Chazy basin. 

 To attain a proper valuation of the bearing of this phenomenon on the 

 paleogeography of the region it becomes necessary to investigate whether 

 this continuity of like conditions in the slate belt is but the result of 

 accidental coincidence or the expression of a general principle. 



With this end in view we will consider the relative lengths of deposition 

 in the other well known graptolite regions, remarking at the outset that the 



