MIDDLE AND. UPPER OEDOVICIAN. 183 



Upon the Maquoketa lies a heavy magnesian limestone of the Niagara, whose basal layers 

 apparently rest concordantly on the magnesian upper Maquoketa beds where these are present. 

 Here the lithologic change is less abrupt than at the bottom of the formation ; nevertheless, the 

 evidence furnished by fossils indicates a hiatus. There is also some evidence that the topmost 

 fossiliferous Maquoketa beds are not everywhere present, this fact suggesting that erosion has 

 actually occurred. 



The Ordovician formations which traverse northeastern Iowa range northward 

 into Minnesota in the apparent embayment of the sea which hes between the Isle 

 Wisconsin ^'^'' on the east and the Peninsula of Minnesota, which terminates in the 

 Sioirx quartzite area, on the west. The strata have been made the object of an 

 elaborate correlative study by Ulrich.^** 



K-L 17-18. ONTABIO AND NEW YORK. 



North of Lake Ontario is a wide belt of Ordovician strata which extends south- 

 eastward around the Adirondack crystalline rocks into the Mohawk and Hudson 

 valleys. New York. In Ontario the section as described by Ami^ comprises lime^ 

 stones and shales from the Potsdam to the Lorraine inclusive. The sandstones are 

 local basal shallow-water deposits resting unconformably on the pre-Cambrian. 



The distribution and character of these strata from Kingston, on the St. Law- 

 rence, to Georgian Bay are described by Logan "^"^ with detailed local sections and 

 hsts of fossils. His general statement runs as follows : 



From the province line, where it crosses Lake Champlain in eastern Canada, the Birdseye, 

 Black River, and, Trenton formations, striking through the valley of this lake and the valleys 

 of the Mohawk and Black rivers, reach the waters of the St. Lawrence and cross over into western 

 Canada. On the United States side of the St. Lawrence they occupy a breadth extending from 

 the Thousand Islands to Sandy Creek, and on the Canadian side from the neighborhood of 

 Kingston to the outside of the Prince Edward peninsula. 



It has already been stated that between the Potsdam formation and the strata character- 

 ized by the fossils of the Birdseye and Black River formations, to the westward of the Laurentian 

 ridge of the Thousand Islands, there are about 80 feet of strata the age of which is not very 

 clear. These appear in two succsediag escarpments and are traceable for a considerable distance. 



Logan then gives a minutely divided section of strata outcropping at Vanluvin's 

 mills, Storrington, which are assigned to the Chazy by Ami. Around Kingston 

 these basal beds are Pamelia, but near Georgian Bay they are Lowville. In 

 either case they are lithologicaUy nearly the same. 



Logan's descriptions of the Black River and Trenton limestones are character- 

 ized by that degree of detail which marks his work and makes it a standard of 

 reference that can not be adequately summarized. 



In New York the Ordovician strata range south of the Adirondack area through 

 Jefferson, Lewis, Oneida, Herkimer, Montgomery, Schenectady, and Saratoga 

 counties to the Hudson. North of the Adirondacks they extend through St. Law- 

 rence, Franklin, and CUnton counties to Lake Champlain. Their distribution as 

 shown on the'map of North America is taken from the New York State map of 1901. ®®^ 



The district west, south, and east of the Adirondacks includes the local sections 

 that were established as the standard New York section, which became the basis of 

 North American correlation. 



