578 



HISTORICAL GEOLOGY. 



848 a 



849. 



Beaohiopods. — Figs. 848, a, Spirifer arenosus ; 849, Eensselaeria ovoides. 



on this page are characteristic ; among Gastropods, a dozen or more species 



of Platyceras; Conidarice, one, C. lata, over five inches long; a few Ortho- 



cerata; Trilobites of 

 tlie genera Iloma- 

 lonotus, Dalmanites, 

 and others. The 

 Homalonotus major, 

 of Whitfield, had a 

 length exceeding 15 

 inches, and a breadth 

 of 5 inches. Dal- 

 manites dentatus, of 

 Barrett, has the front 

 ornamented with a 

 range of large trian- 

 gular teeth, and is 

 the earliest species- 

 of this type of Dal- 

 manites. Acidaspis 

 tuhercidata occurs 



here and also in the Shaly limestone of the Lower Helderberg. 



With the close of the Oriskany period, the Lower Helderberg conditions 



of the Eastern Interior ended. The deposits no longer thickened to the 



eastward. 



Hall remarks on the close relation of the Oriskany fauna in central New York to that of 

 the Lower Helderberg, but in other regions, especially in Ontario and Maryland, to that 

 of the overlying Upper Helderberg. The true Oriskany sandstone or Hipparionyx fauna 

 of New York comprises 45 species (Schuchert), which are chiefly large Brachiopods, 

 Lamellibranchs, and Gastropods, with an almost total absence of Corals and Crustacea. 

 In contrast with this, Beecher and Clarke have shown that the Lower Oriskany fauna of 

 Becrafts Mountain and to the southward contains more than 120 species, of which 15 are 

 Trilobites and about 10 are Corals, and the whole fauna is transitional, showing the pas- 

 sage of the Lower Helderberg fauna into typical Lower Devonian. 



I. C. White concluded, from his observations in eastern Pennsylvania (1882), that the 

 beds were accumulated on the borders of the seas in which the Lower Helderberg lime- 

 stones were at the same time forming in clearer waters, thus making it one with them in 

 period of origin. The beds of the latter often pass directly into the Oriskany, as 

 if they constituted it. In Virginia there is the same close relation to the Lower Helder- 

 berg. It is to be observed, on the other hand, that the beds of Becrafts Mountain overlie 

 those of the Lower Helderberg. At the Delaware Water Gap the rock is largely a shale ; in 

 Maryland, a crumbling sandstone, from loss of its calcareous part ; at Gaspe, a limestone, 

 with probably a part of the underlying sandstone beds, a Bensselceria, having been found 

 1100' above the base of the sandstones. Oriskany fossils are reported also from the head of 

 Tobique River in New Brunswick. The Nova Scotia strata of this epoch occur at Nictaux 

 and on Moose and Bear rivers. They include a thick band of fossUiferous iron ore, which 

 is an argillaceous deposit at Nictaux, but, owing to partial metamorphism, is magnetic iron 

 ore, and partly specular, on Moose River. The Oriskany beds of New York are described 

 in the N. Y. Geol. Bep. of Vanuxem and Hall, in Hall's Fal. Bep., vols. iii. and iv. ; by 



