BIGSBY — PALAEOZOIC ROCKS OF NEW YORK. 339 



east and south, for 400 miles, to Jones ville, on Rock River, near the 

 south boundary of Wisconsin, making the whole northern and 

 western edge 2000 miles long, carefully traced. 



Its eastern boundary is projected, in nearly a direct course, about 

 1 600 miles, from the neighbourhood of Montreal to near Tallapoosa 

 in Alabama, through Vermont, Pennsylvania, and the other more 

 southern States. 



The principal areas of exposure of Potsdam Sandstone are placed 

 at the eastern and western ends of its northern outcrop ; being every- 

 where else covered up, and a mere selvage left, a few miles broad. 



This eastern area, irregular in shape, is very large, and occupies 

 much of the lower parts of the Ottawa Valley (Canada), together 

 with very large portions of the adjacent State, New York (100 m. by 

 60 m.). 



The western mass is still more important, and floors most of middle 

 and western Michigan and of eastern Minnesota. 



In these regions, heavy drift covers up this rock for 730 miles east 

 and west, and 500 miles north and south. These immense spaces, 

 occupied by the ancient sandstone and the boulder-drift, may create 

 a feeling of doubt and surprise ; but it is to be remembered that we 

 are engaged with a continent great in all its developments. 



Position. — Potsdam Sandstone is usually horizontal or undulatory, 

 or with a very small easterly (?) dip, in the State of New York. In 

 the vicinity of intrusive rocks the dip is considerable and various, 

 as to the N.W. at Potsdam, in New York*. 



In De Kalb County this rock is much subject both to lateral and 

 vertical pressure, and in some places is quite broken up ; and there 

 are many faults and uplifts to be noticed in the north and west of the 

 State. (Hall ; Emmons.; 



Thickness. — The thickness of the Potsdam Sandstone is 250-300 

 feet in the N.W. parts of New York, according to Emmons ; 400 

 feet in the S.W. districts, according to Mather ; and 5000 feet, in 

 the estimate of Owen, in the south of Lake Superior (but this re- 

 quires further examination). 



Fossils. — The common fossils are Lingular, Orbiculae, Trilobites, and 

 Scolithus linearis : species few, individuals often numerous. 



Fossils typical. — Lingula prima, L. antiqua (near Depauville), Scoli- 

 thus linearis. Dikelocephalus Meniskaensis, D. granulosus (in micaceous 

 sandstone). D. Minnesotensis (Still-water, Mississippi River, in argillo- 

 and magnesio-calcareous beds). 



Fossils occurrent in Europe. — Scolithus linearis (Stiper Stones, Salter). 



Fossils recurrent in New York. — None. 



Calciferous Sandstone. 



Mineral Character. — This set of strata is intermediate between 

 sandstone and limestone. It occurs in constantly intermingling 

 varieties. 



* For many miles to the south and west of Lake Superior the dip is high, and 

 always to the S.E. (Owen) ; but on the Mississippi, and near the mouth of the 

 Lacroix River — one of its great eastern tributaries, as well as in Wisconsin gene- 

 rally, the strata are nearly horizontal. 



