

WALCorr.] SUMMARY ADIRONDACK SUB-PROVINCE. 341 



of the magnesian limestone Lingula. lamborni, Orthoceras, a turbinated 

 Gasteropod, and a coral, resembling in cross-section a Zaphrentis, were 

 observed. He states that probably this magnesian limestone can also 

 be included in the Potsdam zone. 1 This includes all the limestones up 

 to the base of the recognized third magnesian limestone. 



A more detailed account of this lower marble belt is given by the 

 same writer in a description of the marbles of southeastern Missouri. 

 The most typical section is that of the northern part of Madison County, 

 which is summed up as follows : 



(1) Magnesian limestone. 



(2) Eighteen feet of thick beds of siliceous dolomite and thin shaly limestone, 



with Lingula lamborni. 



(3) Twenty-three feet of gritty dolomite. 



(4) Five to thirty feet of marble. 



(5) Five to forty feet (as much as 90 feet near Mine La Motte) of sandstone. 

 (C) A few feet of slaty sandstone resting on granite. 



The presence of Lingula lamborni in beds a little above the marble will certainly 

 assign the marble beds to the Potsdam group. 2 



EASTERN BORDER OR ADIRONDACK SUB-PROVINCE AND ITS CANADIAN EXTEN- 

 SION. 



The geographic distribution of the Potsdam terrane about the Adi- 

 rondacks is delineated on the geological map of New York accompany- 

 ing the final report in 1842, and that of the Canadian extension on the 

 small map accompanying the 1863 report of Sir W. E. Logan and the 

 large map of Messrs. Logan and Hall, published in 38G6. 3 



On the western side of the Adirondacks the outcropping of the sand- 

 stone is first seen in the northern part of Lewis County, where it rests 

 unconformably upon the subjacent Archean rocks and passes above 

 into the calcareous layers of the Calciferous formation. The outcrop 

 broadens to the north in Jefferson and St. Lawrence Counties and 

 extends around the northern side of the Adirondack area through 

 Franklin and Clinton Counties to Lake Champlaiu. The exposures on 

 the eastern side are more or less interrupted, and rarely occur upon the 

 eastern side of the lake in Vermont, except in Addison and Kutland 

 Counties. On the southwestern side outcrops occur in Washington, 

 Warren, and Saratoga Counties. 



The Franklin and Clinton County area extends across the boundary 

 into Canada, forming quite an extended exposure within the triangle 

 formed by the. St. Lawrence and the St. John's Rivers and the bound- 

 ary, and at one point it crosses to the north of the St. Lawrence Kiver. 

 From St. Lawrence County the sandstone crosses the river and extends 

 along the western side of the Ottawa Basin. 



1 Geological Survey Missouri, including field work of 1873-1874, vol. 1, 1874, pp. 357, 358. 

 2 Marble of Southeast Missouri. Kansas City Review, vol. 5, 1882, pp. 525, 526. 



'Geological map of Canada and part of the United States from Hudson's Bay to Virginia and from 

 the Missouri River to Newfoundland. Montreal, 1866. 



