244 C. SCHUCHERT — LOWER HELDEK&ERG-ORISKANY FORMATIONS 



continuity of faunal evolution. In establishing such a boundary else- 

 where one may assume the physical event to have been general or local 

 in character. In the first case the system boundary is marked by an 

 abrupt change in life ; in the second there will be a gradual faunal transi- 

 tion, necessitating the somewhat arbitrary placing of the boundary. The 

 writer holds that present knowledge is more in harmony with the view 

 that physical events are local and faunas transitional. However, in 

 carrying out this view in correlating widely separated areas, one will incur 

 fewer difficulties by conforming as far as possible to the limitations of 

 the systems as originally defined by their makers. 



The upper limits of the Silurian have been established by Murchison, 

 and strata younger than the " Tilestones " and " Downton Castle sand- 

 stone " can not well be added to this system unless such have a fauna 

 whose facies is Silurian. Professor Williams* writes : 



" Systems were originally groups of successive rock formations ; their limitation 

 was therefore determined, in the first place, by the extent of the rocks in the par- 

 ticular region where they were first defined. Hence the series of formations con- 

 stituting an original system is in each case a standard of reference, and its general 

 application is accomplished by determining its equivalent formations in other 

 regions. . . . 



"It is all-important to know what formations make up these standard systems; 

 for only as other rocks contain the same faunas or floras can they be identified as 

 of equivalent age and therefore as belonging to the same system. The real time 

 indicators are, therefore, the fossils." 



Has the Lower Helderberg a Siluric facies, when but 11 species, or 

 about 2 per cent, of its fauna of 459 forms are derived from the Siluric? 

 The generic differences are even more marked, and Clarke has well said 

 that — 



"The upper limit of the typical Silurian section was clearly defined, and time 

 has shown that this limit was not only one of a geological series, but the dead-line 

 of a large number of organic types." 



As to the relation of the Oriskany to the Lower Helderberg, Hall,t in 

 1859, wrote as follows : 



" The great changes in the physical conditions supervening at the close of the 

 preceding group [Lower Helderberg] indicate an influence which would affect in an 

 equal manner the fauna of the succeeding one, and we find accordingly few species 

 passing from the Lower Plelderberg group to the Oriskany sandstone. The changes, 

 however, are mainly of a specific character; no new genera being introduced. 



" It is not possible, therefore, to point out any changes in the fauna of this period 

 sufficient to indicate the commencement of a new system, and its relations with 



♦ Geological Biology, 1895, p. 31. 



| Paleontology of New York, vol. iii. 1859, pp. 401-402. 



