SUMMARY OF ARGUMENT 341 



which represents the transition from the Ludlow to Devonian of the 

 western sections, but in America we are not left in doubt on this point. 

 The Oriskany has a definite and exact place in the historical succession, 

 and it was during the living of the fauna of the Lower Helderberg stage 

 that the events happened which rapidly modified some and destroyed 

 other species and left a clearly new fauna, first seen in the Oriskany, to 

 dominate the whole eastern border oceans of the American continent. 



Summary of Geological and Stratigraphical Argument 



The geological and stratigraphical argument may be summed up in 

 the following words : A comparison of the geological history of the two 

 continents facing each other, at the point where they most nearly ap- 

 proach each other, shows a general uniformity in the order and sequence 

 of sediments throughout the Silurian. This fact is particularly marked 

 in those points by which the eastern sections differ from those of the in- 

 terior of North America (that is, absence of either an Onondaga basin 

 or a sharp separation of the Niagara and Lower Helderberg faunas). 



This comparison also shows that the geological changes which are 

 expressed in a sharp transition from marine to brackish water deposits 

 terminate the Silurian sections for both continents, and that the place 

 of this transition is definitely located in America where the Oriskany 

 fauna follows that of the Lower Helderberg. The inference is that the 

 place of this transition in America represents, in time, the correspond- 

 ing place of transition in the standard sections in Wales. In Wales this 

 is the boundary plane between the Silurian and the Old Red Sandstone 

 phase of the Devonian system. 



The Paleontological Argument 



NATURE OF THE EVIDENCE 



It remains to show that paleontological evidence confirms this con- 

 clusion. Two cases are in evidence, namely, the recognition of the Tile- 

 stone fauna at the top of the Silurian section at Arisaig, Nova Scotia, 

 and the recent discovery of the same fauna in the Chapman sandstone 

 of northern Maine. 



In both cases the faunas are the most recent Paleozoic marine faunas 

 of their respective sections, and they appear at the point of transition 

 from the typical Silurian into the Old Red Sandstone phase of the 

 Devonian. 



The " Tilestones " have already been referred to as constituting the 

 topmost formations of the typical Silurian system. Their fauna is 



XLIX— Bull. Geol. Soc. Am., Vol. 11, 1899 



