338 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



If the observer sets out from the banks of the Hudson about 

 Albany, his westward course will give him the same undeviating 

 palaeozoic series, and will land him on a grand plateau of carboniferous 

 rocks. 



Premising that we must assume for the present the correctness of 

 the palaeozoic arrangements of our American brethren, as in the main 

 they are, we proceed at once to describe the successive fossiliferous 

 groups, beginning with Potsdam Sandstone, the earliest, as now con- 

 sidered. 



Potsdam Sandstone. 



Mineral character. — It is a purely quartzose sandstone, white, grey, 

 red, and green ; the colour often in stripes and spots. It is frequently 

 also argillaceous and ferruginous by slow and extensive change. 



In some places (Gananoque, St. Lawrence) it assumes the form of 

 a conglomerate of white round pebbles, or of breccia (near Putnam 

 Ferry, Lake Champlain). From its contiguity to igneous rocks, the 

 Potsdam Sandstone sometimes takes on a gneissoid or slaty struc- 

 ture ; or its lower layers are in an intermediate state, crystalline, with 

 rounded gravel or sand intermixed (Mount Stessing). At White 

 Hall, Port Kent, &c, it consists, for small spaces, of the debris of 

 granite or gneiss ; as, besides quartz, we find both felspar and mica 

 in it*. 



Over vast spaces south and west of Lake Superior, Potsdam Sand- 

 stone consists of friable light- coloured quartzose sandstone, with in- 

 tercalations of argillaceous, argillo-calcareous, and earthy deposits. 



Mode of Transition. — Potsdam Sandstone rests unconformably 

 on gneiss (Little Falls, &c.) or other crystalline rock along the 

 Laurentine Hills, a continuation of which is the subalpine country of 

 north-west New York, the planes of contact being commonly abrupt 

 and distinct ; or it may be separated from the subjacent rock by 

 boulders of the latter (Montmorenci, &c). 



On the eastern borders of the basin, as on the frontiers of Vermont, 

 or in New Jersey, Potsdam Sandstone is gradually delivered from the 

 metamorphic condition in which it is found in those countries, and in 

 proportion as it is continued west. 



Place. — This group of strata probably underlies nearly all the 

 central palaeozoic basin. 



Its northern outcrop runs W.S.W. along the south flank of the 

 Laurentine range of metamorphic hills (creeping in among their 

 valleys), from near Quebec to the River Minnesota or St. Peter's, a 

 distance of about 1600 miles. From thence it gradually turns round 



* In Pennsylvania, Prof. H. D. Rogers found this set of strata (the Potsdam) 

 divided into a fourfold group {Johnson's Atlas) : — 



1. Primal, newer slate in Pennsylvania, 700 feet thick. 



2. A white sandstone, 300 feet thick. 



3. Older slate, felspathic and talcose, 1200 feet thick. 



4. Quartzose felspathic conglomerate, with slaty pebbles, 150 feet. 



Dale Owen (Geol. Survey of Wisconsin, p. 52) gives an important account of the 

 mineral characters of the Potsdam Sandstone as it occurs on the Upper 

 Mississippi. 



