334 SCIENTIFIC RECORD FOR 1882. 



Menevian, Potsdam, and Calciferous, including also the Quebec group 

 (Upper Taconic of Emmons). 



The base of the Ordovician, as thus defined, is marked by a great 

 stratigraphical break, attendant on continental movements, in eastern 

 North America. As a result of these conditions, this series, so largely 

 developed in the valley of the Saint Lawrence and the great lakes, rests, 

 as is well known, in many places directly upon the Eozoic crystalline 

 rocks of the Laurentides and the Adirondacks. To the south and east- 

 moreover, the effects are seen in the diminished thickness, in changes in 

 lithological characters, and even in the absence of portions of the series. 

 Examples of these changed conditions are the absence of the Chazy iu 

 localities where the Trenton rests on the Eozoic, as well as in others, 

 where it rests on the Calciferous; and, moreover, in the thinning out of 

 the Trenton itself, and its disai)pearance, or its replacement by argil- 

 laceous beds resembling those of the Utica or of the underlying Que- 

 bec group, as noted by Whitfield and others in the vicinity of Albany. 

 Similar strata of Ordovician age, as long since pointed out, are found 

 in eastern Canada, apparently dipping beneath the older Cambrian 

 (Quebec group), in which they were by Logan included. Eecently 

 Selwyn has found iu this region portions of these newer fossiliferous 

 strata lying to the east of a belt of crystalline Huronian rocks, and rest- 

 ing directly upon the latter. Such outliers of Ordovician strata, with 

 fossils of Trenton or Utica age, have now been found in several places 

 among the crystalline schists of the Green Mountain belt in the prov- 

 ince of Quebec, in connection with lines of fault and downthrow, which 

 have protected these newer strata from erosion. Eecently, also. Dodge 

 has found graptolitic slates, referred to this horizon, in Penobscot 

 County, Maine. 



These, and other similar facts show the former extension, under more 

 or less modified conditions, of Ordovician rocks over the Cambrian, and 

 still older series to the south and east of the Saint Lawrence, Champlain, 

 and Hudson valleys. The great belt of uncrystalline sedimentary rocks 

 stretching throughout these regions along the western base of the crys- 

 talline range was by Mather, and later by Logan, described as the Hud- 

 son Eiver group, and supposed to belong to a horizon above the Trenton 

 limestone; while by Emmons, who subsequently assigned it to a posi- 

 tion below this limestone, it was called Ui^per Taconic, and afterwards 

 was by Logan, who adopted the view of Emmons, named the Quebec 

 group. Researches in Canada and iu Vermont have long since shown 

 that iu this greatly disturbed and involved belt are included fossiliferous 

 strata, holding all three of the great lower Paleozoic faunas, Cambrian, 

 Ordovician, and Silurian. 



The rocks of the so-called Hudson Eiver group, near Poughkeepsie, 

 N. Y., have recently been made the subject of studies by Dale, Whit- 

 field, Dana, and Dwight, with the result of discovering there fossilif- 

 erous beds referred to the various horizons of the Loraine, Trenton, 



