POTSDAM GROUP. 29 



lent in part of the Taconic System. It was not used in the sense of a "Group," 

 but in the higher sense of a "System," as these words were then understood. It 

 was never well defined, and it crossed one of the grandest and most important 

 breaks in geological time — that which separates the Taconic and Silurian Systems. 

 No careful geologist or palaeontologist uses the word in the nomenclature of American 

 strata, though it occasionally occurs in incoherent geological papers, and some- 

 times we see such monsters in nomenclature as Cambro-Silurian and Siluro- Cambrian. 

 § 54. The Lower Silurian in North America is divided, in ascending order, into 

 the following Groups: viz., Potsdam, Calciferous, Quebec, Chazy, Black River, 

 Trenton, Utica Slate, and Hudson River. The Upper Silurian is divided, in ascend- 

 ing order, into the Medina, Clinton, Niagara, Onondaga, Guelph, and Lower 

 Helderberg. 



POTSDAM GROUP. 



§ 55. Prof. Ebenezer Emmons, in the Annual Report of the Geological Survey 

 of New York for 1838, described the sandstone at Potsdam in St. Lawrence County, 

 and proposed for it the designation " Potsdam Sandstone." It was subsequently 

 described quite fully in the New York Reports, and finally the Canadian Geolo- 

 gists in 1863 called the rocks the Potsdam Group. The lowest portion at Potsdam 

 is a granitic conglomerate, in which large masses of quartz, the size of a peck meas- 

 ure, are sometimes inclosed. These were water-worn and rounded before being en- 

 veloped in the deposit. The sandstone is quite variable in texture and color, but its 

 composition is uniformly silicious. At some places it is an even-grained mass in 

 compact layers, and at others it is traversed by joints. In some localities a dark, 

 slaty sandstone, about ten feet in thickness, intervenes between the Potsdam and 

 Calciferous, at others a coarse brecciated rock, and at others the passage is very 

 gradual into the Calciferous sand-rock. The thickness in New York is from 100 to 

 200 feet. The exposure is narrow, but extends from near the Thousand Islands 

 to Lake Champlain, and enters Vermont with a thickness of about fifty feet. 



§ 56. It extends from New York into Canada, where it attains a thickness 

 ranging from 300 to 700 feet, and at the summit the sandstone is interstratified 

 with magnesian limestone that constitutes a passage to the Calciferous. There is 

 more diversity in the rocks in Canada than in New York, and limestones and slate 

 sometimes occur with the sandstone. It rests unconformably upon the Laurentian, 

 and fills up inequalities where the Taconic System does not intervene, and it also 

 rests unconformably upon the Taconic when it is present. The sandstone appears 

 to have been deposited in shallow water along the margin of a sea. The tracks 

 and wind marks support that view. In its extension westerly, by the way of 

 Lake Huron and Lake Superior, across Wisconsin and into Minnesota, the same 

 variations in thickness occur. Sometimes it attains a thickness of 3,000 feet, and 

 again thins out to 40 or 50 feet. For several miles in distance near Beauharnois, 

 Canada, the strata are marked by the tracks of Protichnites. The surfaces on 

 which the tracks are impressed are sometimes smooth, and sometimes beautifully 

 ripple-marked. On the latter the tracks have often beaten down the ripple- 

 marks, and the sand of the ridge has been dragged into the furrow, in such a way 

 as to show the direction in which the animal was progressing. Fucoids are abun- 

 dant in the upper part of the Group, and Scolithus so common as to be quite 



