104 HISTORICAL GEOLOGY 



importance, so that few horizons are missing. As was stated in 

 connection with the Ordovician, so here, it should be remembered 

 that many formation or stage names have been more or less locally 

 applied in North America to formations not yet definitely cor- 

 related with those of New York, or to a few others not represented 

 in New York. Also the lithologic character of formations may be 

 quite different in different regions. 



Distribution and Character of the Rocks 



General Distribution. — The present surface distribution of the 

 Silurian rocks in North America is shown on map, Fig. 59, which 

 is largely self-explanatory. Certain points of comparison with the 

 Ordovician (see map, Fig. 41) need to be mentioned. Thus to a 

 very considerable degree the Silurian and Ordovician rocks occur 

 in the same areas, the chief differences being much more extensive 

 areas of Silurian strata in the Arctic Islands region, their almost 

 complete absence from the upper St. Lawrence Valley, and their 

 much smaller representation in the mid-Mississippi Basin, Rocky 

 Mountains, and Great Basin of the west. 



As stated in connection with the two preceding areal distribu- 

 tion maps, so here, the surface distribution of Silurian rocks by no 

 means indicates the former or even present actual extent of these 

 rocks in North America. From many regions Silurian strata have 

 been removed by erosion, while in other regions they are concealed 

 under cover of later rocks. Thus most of the upper Mississippi 

 Basin, with its essentially horizontal strata, is underlain with 

 Silurian rocks, and only the eroded edges of upturned Silurian 

 strata are exposed in the Appalachian Mountains. 



The Oswegan Series. — This series, in the northeastern United 

 States, consists principally of the Oswego sandstone, and Medina 

 sandstone, shale, and (Oneida) conglomerate. Ripple-marks, 

 cross-bedding, and the character of the fossils prove these to have 

 been deposited in a very shallow, probably encroaching, sea. The 

 Oneida conglomerate is made up of well-rounded pebbles, bears 

 all the marks of a typical marine-beach or very shallow-water 

 deposit, and in central New York rests upon the eroded edges of 

 the Upper Ordovician shales. 



The Niagaran Series. — This series is of special interest both 

 because of its lower or Clinton beds and its higher or Lockport and 



