346 IT. S. WILLIAMS — SILURIAN-DEVONIAN BOUNDARY 



sandstone, in the report on the geology of Maine above referred to. The 

 identification of this fauna with the Tilestone fauna of Murchison was 

 made before I observed the fact that Salter had positively identified 

 Honeyman's Arisaig fauna D as equivalent to the Tilestone. In pre- 

 senting the facts before the Society, I have therefore given precedence 

 to the Arisaig facts, and I omit the paleontological argument by which 

 the correlation of the Chapman and Tilestone fauna is established, which 

 will be published hereafter.* The species of the Chapman sandstone 

 and the upper Arisaig are, in several cases, identical, so far as I can judge 

 from reading the descriptions, and the whole contents of the two faunas 

 are very much alike. The Chapman sandstone fauna is the latest of 

 the marine Paleozoic faunas of Maine, and, what is still more significant, 

 a Psiloplyton stem is found associated with the marine fossils in the 

 Chapman sandstone. In the Chapman sandstone there are a few species 

 with which to correlate the fauna with another, found in western Maine, 

 about Moose river, in which a distinctly Oriskany fauna is recognized. 

 This shows that the Chapman fauna is closely related to the Lower 

 Oriskany of New York. In Maine (Aroostook county), the Square Lake 

 limestone, which is correlated with the upper part of the Gaspe lime- 

 stone, contains a distinctly Lower Helderberg fauna, thus establishing 

 for the Lower Helderberg a place in the scale earlier than the represent- 

 ative of the Welsh Tilestone (my Chapman sandstone), and hence un- 

 questionably in the typical Silurian system. 



Summary and Conclusions 



Thus, the Arisaig, the Gaspe, and the Maine sections are in harmony 

 in fixing the exact boundary between the Silurian and Old Red sand- 

 stone (Devonian) on the American continent. As near as I am at present 

 able to identify this boundary in the New York sections, it comes very 

 close to the transition from the Lower Oriskany of Becraft mountain into 

 the Upper, or pure Oriskany of Oriskany falls. This point will be de- 

 veloped as the faunas are more full}' studied. The facts already noted, 

 however, are clear in establishing the fact that the Lower Helderberg 

 fauna lies below the Chapman sandstone, and the equivalent upper 

 Arisaig, faunas, both of which are the paleontological representatives on 

 tins continent of Murchison's Tilestone fauna (Downton sandstone and 

 Ledbury shales), the highest known fauna of the typical Silurian system. 



*See Am. .lour. Sci., vol. ix, p. 203. 



