232 CONFERENCE ON PALEOZOIC PALEOGEOGRAPHY 



The combined facts of the world-wide distribution of the more com- 

 mon graptolites and of their planktonic mode of life, together with the 

 restriction of the graptolites to either the edges of continents or former 

 deep submarginal troughs, lead to the conclusion that the home of the 

 graptolites was in the practically permanent oceanic basins, and that they 

 were but strangers in the relatively occasional epicontinental seas. 



This conceded, it further follows that these oceans were connected, and 

 that the graptolite horizons of world-wide distribution indicate s3^nchrony 

 and not merely homotaxy, and thus are probable means of world-wide 

 correlation; further, that where the graptolites are found in a series of 

 zones they indicate near-oceanic conditions — that is, proximity to an 

 ocean — the presence of oceanic currents, which carried these planktonic 

 organisms through the basin, and possibly, also, a greater depth of water 

 than usually occupied continental basins and troughs. 



Each of these conclusions requires, again, some qualification and 

 9xplanation. 



First, although a certain percentage of the species in every zone may 

 be world-wide, others differ sufficiently to warrant the recognition of pro- 

 vincial features also among the graptolites. The minor provincial differ- 

 ences correspond apparently to the opposite sides of oceanic basins, the 

 larger ones to the different oceans themselves. It follows from this that 

 where the latter differences are pronounced, as in part of Ordovicic time, 

 that the oceans must on one hand have been in sufficient intercommunica- 

 tion to permit world-wide distribution of the common graptolites by the 

 currents, and still sufficiently defined and separated to also favor the 

 development of provincial characters in the plankton, or, in other words, 

 the relative areas of water and land were not materially different from 

 the present condition. 



The inference that the graptolites were but strangers in the epiconti- 

 nental seas explains the observation that they occur there only sporadic- 

 ally — as, for instance, the zone of Monograptus dintonensis and Retio- 

 graptus venosus, in the upper Williamson shale of the Clinton of western 

 New York. Where graptolite faunules appear as abruptly and for such 

 a brief period only, as in the Williamson, it is safe to say that this incur- 

 sion is caused by the breaking of an ocean current through a barrier, and 

 its free, though short-lived, passage through the basin. In the case of 

 the Williamson shale I am convinced that the eastern embayment of the 

 Mississippi basin, mapped by Professor Schuchert for the Wolcott- Wil- 

 liamson stage, connected at this brief stage with the Appalachian basin, 



