252 C. SCHUCHEKT — LOWER HELDERBERG-ORISKANY FORMATIONS 



From Lower Helderberg time deposition is again continuous, and the 

 chain of life in the American Appalachian and Mississippian seas is 

 almost unbroken to the close of the Paleozoic. There is probably no 

 land area preserving so continuous and undisturbed a sequence of Paleo- 

 zoic deposits as that of eastern North America. From Lower Cambric 

 time to the close of the Permic, the only marked hiatus exists between 

 the Ordovicic and Siluric, and this may have been caused by the eleva- 

 tion of the Taconic mountains. However, beyond this area, in the gulf 

 of Saint Lawrence, even this gap will probably be largely bridged over 

 by the fossils preserved in the Ordovicic and Siluric strata of Anticosti 

 island. These, as }'et, have not been sufficiently studied ; but it is clear 

 from the work of Billings that the Ordovicic and Siluric fossils of 

 Anticosti have more in common than these S}' , stems have elsewhere in 

 America. 



The table on page 251 shows the equivalents of the English Upper 

 Silurian as understood by Murchison in 1854, and by Geikie and Kayser 

 in 1893, with the American homotaxial equivalents.* 



CONCL USIONS 



With the foregoing presentation, it is clear that the upper faunal limits 

 of the original Upper Silurian have been and still are vague, because the 

 normal marine fauna gradually succumbed to local conditions associated 

 with the production of red sediments. 



"As a series of rocks 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." f 



Lower DevonicJ 

 devonian of sedgwick and murchison 



The following resume by Murchison § gives a clear history of the 

 progress regarding the Devonian rocks in the type area; 



"Devonian rocks {the equivalents of tJte Old Red) in Devon and Cornwall. — The crys- 

 talline and slaty condition of most of the stratified deposits in Devon and Cornwall, 

 and their association with granitic and eruptive rocks and much metalliferous mat- 



*The divisions and terminology here adopted are those of Clarke and Sehuchert. Science, 

 vol. x, December 16, 1899, pp. 874-878. 



fClarke: Review of "Die Fauna des Unteren Devon am Ostabhange des Ural." Am. Geo!., 

 vol. xiv, 1894, p. 121. 



{The results of this section are arranged in tabular form beyond. 



gSiluria, London, 1854, pp. 257-258. For an encyclopedic paper on the history of the Devonian 

 to 1867, st«' Quart. Jour. Geol. Soc. London, vol. xxiii, p. 568, by Robert Etheridge ; also "The 

 Geology of England and Wales," )>y Horace B. Woodward, London, 1887. 



