12 



Utica Slate and 



hundred species known to the writer as occurring therein, fifty-four 

 are peculiar to it and not known above or below its horizon. The 

 graptolites are the most constant forms but over a great area the 

 Triarthrus Becki is only second to them. The few individuals of 

 the species of trilobite that have been found in the Trenton lime- 

 stone were in the upper shaly limestone, which was deposited when 

 the conditions bringing about the Utica slate deposit were being in- 

 troduced. 



Triarthrus Becki and the various forms of graptolites appear to 

 have been, notwithstanding their fragile character, peculiarly adapted 

 to spread over an extended area in the muddy bed of the sea, while 

 the clearer limestone-forming seas were not favorable to the develop- 

 ment of the trilobite and to but few species of the graptolites ; 

 the trilobite has not been found to the knowledge of the writer to the 

 north-west, west, or south of the Cincinnati exposure. That this tri- 

 lobite and the graptolites should have obtained so wide a geographi- 

 cal distribution is evidence of the comparative slow deposition of the 

 sediments forming the Utica slate ; this view is also strengthened 

 by the presence of the large and fully developed Asaphus Halli and 

 Asap/ius Canadensis at CoUingwood, where the shiales are filled with 

 their remains and those of graptolites, brachiopods and orthoceratites. 



The return of conditions favorable to the existence and extension 

 of the graptolites and Triarthrus JBecki, at the commencement of 

 the deposition of the Hudson River formation in Illinois, Wisconsin 

 and Iowa, would have undoubtedly led to their extension to the 

 west, either by the way of the central basin or the northern coast 

 line, had they not been replaced by the fauna of the later beds at 

 Cincinnati. I would regard the deposition of the sediment forming 

 the Galena limestone as going on during and after the deposition of 

 the Triarthrus beds at Cincinnati ; the calcareous marls and sedi- 

 ments of the Hudson River formation over the central basin, to the 

 west of the extension of the Utica formation, gradually forcing their 

 way, towards the close of the epoch, over the Galena limestone- 

 formino" area, and eventually burying it beneath a deposit of shales 

 and preventing for a time the deposition of the sediments forming 

 the magnesian limestones, so characteristic of the western side of 

 the Appalachian basin. 



The shales resting on the Galena limestone in Wisconsin and 

 Iowa, cannot, in the writer's view, be considered as the possible or 

 probable equivalent of the Utica slate. There is no similarity in the 

 fauna and the fact of their being somewhat carbonaceous is not 

 sufficient to correlate them. 



