248 C. BCHUCHERT — LOWER II ELDERBERG— ORISK AN Y FORMATIONS 



" Downtonian has hitherto been regarded as of Lower Old Red Sandstone age, 

 owing to the prevalence of red and yellow .sandstones and shales which are the 

 prominent feature of that formation. The recent discovery by the Geological Sur- 

 vey, in shales and mudstones intercalated in these sandstones, of a marine fauna 

 which in some respects is identical with that of the underlying Ludlow Rocks has 

 led to a revision of the classification hitherto adopted. These passage beds are now 

 viewed as forming the highest subdivision of the Upper Silurian rocks." 



The following is a tabulation of the Downtonian, in descending order, 

 with an enumeration of the essential fossils. The thickness of the series 

 is from 700-2,700 feet. 



4. Chocolate-colored sandstones. 



3. Conglomerate with quartzite pebbles derived from the Highlands. 



2. Green and red mudstones with bands of gray wacke and brown iiaggy carbona- 

 ceous shales with fishes and eurypterids. Plants: Pachytheca, Parka. Ani- 

 mals: Beyrichia, Ceratiocaris, Eurypterus, Pterygotus, Slimonia, Stylonurus, 



Glauconome, Spirorbis. Fishes : Thelodus and four other restricted genera. 



1. Red and yellow sandstones and mudstones. 



i 



The fossils mentioned above are Siluric in age and remind rather of 

 the Tentaculite and Waterlime than of the Lower Helderberg. Messrs 

 Peach and Home add : 



" In view of the paleontological and to some extent also of the physical evidence 

 regarding the passage-beds that overlie the Ludlow rocks, there seems ground for 

 maintaining that they have greater affinities with the Silurian system than with 

 the Old Red Sandstone. They may be looked upon as stratigraphical equivalents 

 of the Tilestones, Down ton sandstones, and Ledbury shales, which in Hereford- 

 shire, overlie the Upper Ludlow rocks, and have been classified as forming the 

 highest subdivision of the Upper Silurian rocks."* 



.4 M ERICA N EQUIVALEN TS 



In North America the Siluric faunas are also abundant in species, and, 

 with a gradual change toward higher types, continue from the Anticosti 

 formation through the Guelph, when red shales, salt and gypsum bear- 

 ing, begin and bear witness to an almost total extermination of animal 

 existence throughout the Onondaga (= Cayugan) formation. Here, as in 

 England, eurypterid crustaceans are the prevailing fossils, but in America 

 fishes are generally absent. 



In eastern New York, above the " Waterlime " a small but normal 

 marine fauna again makes its appearance in the Tentaculite (= Manlius) 

 limestone. It consists of 26 species, and 4 of these, are known to occur 



* Silurian Rocks of Great Britain, p. 5G8. 



