GRAPTOIJTES OF NEW YORK, PART 1 493 



Britain to be regarded as marking the boundary between the Cambric and 

 Siluric eras. 



From the material collected by the geologists who have been engagta in 

 mapping the slate belt of New York and Vermont, I infer that both D i c t y - 

 onema flabelliforrae and Stanrograptus dichotoraus are 

 also present at other localities in the slate belt of New York, notably in the 

 region of Granville in Washington county [see localities under D i c t y - 

 onema flabelliforme, p.601]. The fact that almost without exception 

 only the more common early growth stages had been collected, prevented the 

 recognition of the presence of this stratigraphically important zone in the 

 siate belt by the paleontologist of that survey. Subsequent investigation of 

 the outcrops of the horizon in Washington county by the present writer has 

 brought out the fact that the Dictyonema shale is found infolded, northward 

 from the Hoosic river to and beyond the Vermont boundary, between the 

 Georgian shales and slates and the basal rocks of the Lower Siluric, notably 

 the thinly bedded limestones and intercalated shales, characteristic of the 

 Beekmantown graptolite horizons at the Deep kill. 



There is no doubt that the zone extends into Vermont, for Dicty- 

 onema flabelliforme is, for example, found at Fairhaven Vt. and 

 probably it extends northward through that state and merges into the belt of 

 Dictyonema slates skirting the south shore of St Lawrence bay. 



An interesting fact brought out by the presence of the shale with 

 Dictyonema flabelliforme in the slate belt of eastern New York, 

 one which has an important bearing on the paleography of eastern North 

 America, is that, while the contemporaneous Upper Cambric or Saratogian 

 littoral facies of New York — the Potsdam sandstone, Greeniield limestone, at 

 Saratoga etc. — by their included Dicellocephalus fauna were clearly deposited 

 in the American Pacific basin, the Atlantic waters encroached close by on the 

 present territory of the slate belt, probably along the "Levis channel" 

 outlined by Ulrich and Schuchert, for the earlier Lower Siluric. [See 

 appended chart on the distribution of the fauna with Dictyonema 

 flabelliforme. 



