BIGSBY — PALAEOZOIC ROCKS OF NEW YORK. 353 



upper terminal of the Hudson-River-group in Oswego County (Hall, 

 Rep. p. 31). The two intermingle, until the Medina group, always 

 distinguishable by its red colour, is gradually substituted. There 

 is, in truth, in central New York, a slow passage from the Hudson- 

 River-group to the Medina, — a fact which explains the escape of 

 certain fossil species from the lower to the upper Silurian stage by a 

 lateral propagation of life. 



Thickness. — I have few materials on this head. At Rochester, 

 more than 100 feet are exposed; and 60-65 miles west, about 

 Queenston, the thickness may be 350 feet (Hall). 



Fossils. — This group, throughout the greater part of its thick- 

 ness, is almost devoid of fossils. Towards the top of the mass (in 

 grey quartzose sandstone No. 2), where it is more arenaceous, in 

 several places (Oswego, Rochester, Medina, Lockport, especially) it 

 contains many fossils, though comprehending few species. Between 

 this point in Medina Sandstone, where fossils appear, and the point 

 in the Hudson-River-group where similar fossils are found, there is 

 a thickness, in some parts, of 1000 feet where no well-defined forms 

 are known to exist. Thus, the grey sandstone ending the Hudson- 

 River-group, the Oneida Conglomerate, and the lower part of the 

 Medina Sandstone, separate the fossiliferous portion of the Hudson- 

 River-group from the point with new organisms, where we are now, 

 by a great thickness of barren strata. It is true that in the east 

 part of the State these groups approach each other within 100 feet 

 by thinning ; but there also is no life. 



The fossiliferous portion of the Medina Sandstone (the upper) is 

 not thick ; but at the west frontier of the State it attains a thick- 

 ness of 100 feet. 



Medina Sandstone only receives four forms of life from previously- 

 existing strata ; and it transmits two, perhaps only one. 



The Tables II. & IV. show the number of species and the kinds of 

 animal life which escape into and through the disturbed transition- 

 period and past the Oneida Conglomerate. Most of them are recur - 

 rents, and some travel great distances. 



Fossils typical. — These are seventeen in number, and comprehend 

 neither Zoophyte, Bryozoan, Echinoderm, nor Trilobite. 



Fossils occurrent in Europe. — Cytherina cylindrical only. 



Fossils recurrent in New York — Bucania trilobata and Calymene 

 JBlumenbachii. 



The peculiar marine plants of this sandstone are a marked feature 

 (Hall, Pal. ii.) . They extend into the Clinton group, and are seen 

 nowhere else. This observation applies equally to New York, Penn- 

 sylvania, and Virginia. A belief, however, is gradually arising that 

 many of these supposed plants are in reality Annelida. 



Clinton Group. 



Mineral Character. — The lower part of this set of strata has 

 been well named the Protean Group. It consists of alternating 



* Leperditia cylindrica, Hall sp., Jones, Annals & Mag. Nat. Hist. 3rd ser. 

 vol. i. p. 253. 



