538 HISTORICAL GEOLOGY. 



The conditions described and illustrated on the map make it apparent 

 that the Interior Continental Sea opened westward toward the Pacific, but 

 not eastward over temperate latitudes toward the Atlantic ; and hence that 

 migrations to those seas from Eurasia should have beea chiefly from the 

 west rather than from the east. On the contrary, New England and eastern 

 Canada remained still open toward the Atlantic and. Europe, and hence dif- 

 ferences in the cotemporaneous faunas of this eastern part of North America 

 and of the Continental Interior should be expected. 



1. Niagara Period, 

 rocks — kinds and distribution. 



The beds over New York referred to the Medina epoch — the earlier part 

 of the Niagara period — include, to the eastward, a seashore formation, called 

 the Oneida conglomerate from Oneida in central New York, and an off- 

 shore sand-flat formation, called the Medina sandstone from Medina in the 

 western part of the state. 



The Oneida conglomerate is a hard light-gray rock made of quartz 

 pebbles and sand. It covers large areas in Oneida and Herkimer counties, 

 N.Y., but thins out eastward to 15 to 20 feet at Kondout on the Hudson. 

 It comes up again in Ulster County (southeastern New York), owing to an 

 uplift along the Shawaugunk (pronounced Shong-gum) Mountains, and is 

 there called the Shawangunk grit. This range commences near the Hudson, 

 southwest of Kingston, and to the southwest, between New Jersey and Penn- 

 sylvania, becomes the Kittatinny Mountains. The grit makes the crest and 

 southwest front of these mountains, and the beds dip westward 30°-40°. 

 Thence the conglomerate, or grit, stretches on southwestward through Penn- 

 sylvania into Virginia, where " it makes the bony axis of the principal Appa- 

 lachian ridges" (Rogers), and beyond into east Tennessee, Avhere it is the 

 Clinch Mountain sandstone of Safford. In Ulster County, N.Y., near Red- 

 bridge, the Shawangunk grit has afforded galena and copper pyrites in large 

 masses, and fine crystals; but the mine is not now worked. The Medina 

 sandstone is ordinarily a gray to red mottled sandstone, fine-grained, thin- 

 bedded, somewhat argillaceous, especially so to the westward, and bears 

 evidence of having originated as a great sand-flat formation in shallow waters, 

 as first described by Hall; for its layers are often covered with ripple- 

 marks, wave-marks, and rill-marks, evidences of exposure above the waters, 

 perhaps with the retreat of the tide, and in many places of gentle wave 

 action on a slightly inclined beach. In making the rill-marks (page 95), 

 the retreating undertow swept past worn shells of Lingula cuneata or small 

 pebbles in the surface sands of the beach. 



The beds are not found in eastern New York near the Hudson, but 

 mainly to the west of Oneida County. They border Lake Ontario to its west- 

 ern extremity, and constitute the lower half of the Niagara bluffs at Lew- 



