Geological Society of London. 515 



The single species, which is named S. Nicholsoni, is from calcareous 

 shale of Lower Helderberg (Ludlow) age, near Dalhousie, in New 

 Brunswick. 



8. " On the Superficial Geology of the Central Eegion of North 

 America." By G. M. Dawson, Esq., Assoc. B.S.M., Geologist to 

 H.M. North American Boundary Commission. Communicated by 

 Dr. Bigsby, F.B.S., P.G.S. 



Physical geography of the region. — The region under consideration 

 is that portion of the great tract of prairie of the middle of North 

 America from Mexico to the Arctic Sea, which lies between the 

 49th and 55th parallels, and extends from the base of the Bocky 

 Mountains to a ridge of Laurentian rocks that runs N.W. from Lake 

 Superior towards the Arctic Seas, and is called by the author the 

 " Laurentian axis." 



This plateau is crossed by two watersheds ; one, starting from the 

 base of the Bocky Mountains at about the 49th parallel, runs due 

 east to the 105th meridian, when it turns to the S.E., dividing the 

 Bed Biver from the Missouri ; the other crosses from the Bocky 

 Mountains to the Laurentian axis near the 55th parallel. The 

 whole region between these two transverse watersheds slopes gra- 

 dually eastward, but is divisible into three prairie steppes or plateaus 

 of different elevations. The lowest includes Lake Winnipeg and the 

 valley of the Bed Biver ; its average altitude is 800 ft. The second, 

 or the " Great Plains," j)roperly so called, has an average ele- 

 vation of 1600 ft. The third or highest is from 2500 to 4200 ft. 

 above the sea, and is not so level as the other two. 



Glacial phenomena of the Laurentian axis. — The neighbourhood of 

 the Lake of the Woods is taken by the author as furnishing an 

 example of the glaciation visible in many parts of the Laurentian 

 axis. This lake is 70 miles long, and has a coast line of 300 or 400 

 miles. The details of its outline closely follow the character of the 

 rock, spreading out over the schistose and thinly cleavable varieties, 

 and becoming narrow and tortuous where compact dioritic rocks, 

 greenstone, conglomerate, and gneiss prevail. The rocks both on 

 the shores and the islands in the lake are rounded, grooved, and 

 striated. The general direction of the striae is from N.E. to S.W. 



Drift Plateau of Northern Minnesota and Eastern Manitoba. — This 

 plateau consists of a great thickness of drift deposits, resting on the 

 gently sloping foot of the Laurentian, and is composed to a depth of 

 60 feet or more of fine sands and arenaceous clays, with occasional 

 beds of gravel and small boulders, probably reposing throughout on 

 boulder-clay. The only fossil found was a piece of wood apparently 

 of the common cedar (Thuja occidentalis) . 



The surface of the plateau is strewn with large erratics, derived 

 chiefly from the Laurentian and Huronian to the north ; but there 

 are also many of white limestone. The fossils in some of the latter 

 being of Upper Silurian age, the author is inclined to believe, with 

 Dr. Bigsby, that an outcrop of Upper Silurian is concealed by the 

 drift deposits in the Lake of the Woods region. 



Lowest Prairie Level and Valley of the Bed Biver. — This prairie 



