8 NEW YORK STATE MUSEUM 



LOCATION AND CHARACTER 



The district here reported upon comprises the Brier Hill, Ogdens- 

 burg and Red Mills quadrangles of the topographic map sheets. 

 The southwestern margin of the Brier Hill sheet overlaps by a 

 trifle the northeastern corner of the Alexandria Bay sheet, so that 

 the mapping is a continuation, down the river, of the work done in 

 the Thousand Islands region. 1 In the latter territory the Paleozoic 

 rocks found are chiefly those to the south of the Frontenac axis, 

 as the belt of crystalline rocks which comes down to and crosses 

 the river at the Thousand Islands is called. In the Ogdensburg 

 district the Paleozoics are those to the north of this axis, and the 

 contrast between the two will be subsequently shown. 



The mapped area extends from longitude 75 15' to 75 45' W. 

 and from latitude 44 30' to the St Lawrence river. It is of tri- 

 angular shape, since the river flows northeast. At the west edge of 

 the Brier Hill sheet the river crosses the parallel of 44 30' ; at the 

 east edge of the Red Mills sheet the south bank of the river is about 

 at latitude 44 52'. The area included is about 320 square miles. 



The district lies entirely in the topographic province of the St 

 Lawrence plain, though its southern margin might be said to belong 

 to the northwest edge of the Adirondack highland. On the north- 

 west there is not the sharp junction between these two topographic 

 provinces that there is farther east, but a very gradual drop from 

 the one to the other. 



The district lies also on the boundary between two geologic 

 provinces, which correspond in a general way with the topographic. 

 The crystalline rocks of the Adirondack highland descend to low 

 levels in this vicinity, and cross into Canada at the Thousand Islands 

 in a narrow belt, furnishing an isthmianlike connection between the 

 great area of these rocks in the Adirondacks, and the vastly greater 

 area in Canada. Below the Thousand Islands the river flows 

 through a country of low altitude whose rocks are flat-lying forma- 

 tions of early Paleozoic age, and crystalline rocks do not reappear 

 along the river west of Quebec. The general breadth of this Paleo- 

 zoic plain of the St Lawrence valley, which separates the Adiron- 

 dack highland from the Canadian highland, is from 60 to 70 miles, 

 but three-fourths of this breadth, and of the plain, lies on the 

 Canadian side of the river. 



N. Y. State Mus. Bui. 145. 



