Br. Bigsby on the Palceozoic Strata of the State of New York. 77 



heads above referred to, the following being the order of classifica- 

 tion used by the author. 



A. Lower Silurian : 1. Potsdam sandstone ; 2. Calcifcrous sand- 

 stone ; 3. Chazy limestone; 4. Bird's-eye limestone; 5. Trenton 

 limestone; 6. Utica slate; 7. Hudson's River rocks. B. Middle 

 Silurian, or Transitional group : 8. Oneida conglomerate ; 9. Medina 

 sandstone; 10. Clinton rocks. C. Upper Silurian: 11. Niagara 

 shales and limestone ; 12. Coralline limestone ; 13. Onondaga salt 

 rocks; 14. Waterrae rocks; 15. Lower Pentamerus limestone; 

 16. Deithyris shaly limestone; 17. Upper Pentamerus limestone. 



D. Lower Devonian : 1. Oriskany sandstone ; 2. Cauda-gaili grit ; 

 3. Schoharie grit; 3. Onondaga limestone ; 4. Corniferous limestone. 



E. Middle Devonian: 5. Marcellus shale ; 6. Hamilton rocks; 7. 

 TuUy limestone; 8. Genessee slate; 9. Portage sandstone; 10. 

 Chemung rocks. F. Upper Devonian: 11. Catskill or Old Red 

 Sandstone. 



From the consideration of the many members of this great palseo- 

 zoic series of Central North America, severally and collectively, the 

 author drew numerous inferences, and offered them to the notice of 

 the Society, not as all being new or unassailable, but as affording 

 many points of interest both for geologists in general, and the 

 student of the American rocks and fossils in j)articular. 



The following are some of the principal inferences : — That the 

 Silurian and Devonian Systems of New York belong to one con- 

 nected period ; being the products of successive and varying Neptu- 

 nian agencies, operating in viaters which deepened westward from 

 the Atlantic side, and southward from the Laurentine chain on the 

 north. 'I'hese palaeozoic groups pass one into the other by gradual 

 mineral and zoological changes ; there being a neaily perfect conform- 

 ability and a considerable community of fossils. The chief break is 

 at the Oriskany sandstone, there being no break of like importance at 

 the period of the local Oneida conglomerate. A division of the 

 Silurian and Devonian systems, each into three stages, is based on 

 change of sediment and the fossil contents. The Middle Silurian 

 stage is a period of especial transition ; from the coarseness of some 

 of its sediments, from their alternations, and from the organic poverty 

 prevailing. The New York Basin exhibits few uplifts ; it consists of 

 a number of comparatively thin undulating sheets of sediments (about 

 13,300 feet thick altogether) dipping slightly to the south-west, here 

 and there pierced by a peak of crystalline rock, and along three lines 

 raised into broad low domes of great length. The strata have ex- 

 perienced two kinds of disturbance, from plutonic agency : 1. secular 

 or slow oscillation during deposition ; 2. i)aroxysmal uplifting sub- 

 sequent to their formation, and probably of j)ost-carboniferous date. 

 The form and di.-jction of the live great Canadian Lakes follow the 

 outcrops of the local sedimentary rocks ; and the contour of the 

 valley of the St. Lawrence is also due to the relative positions of these 

 palaeozoic strata. Some of the groups of strata, during and after 

 dejjosition, were subaiirial, presenting the conditions of dry land 

 and shallow waters, for long and varying periods. In New York, as 



