INDUSTRIAL PROGRESS DURING THE YEAR 1873. xlvii 



]y hydrated, in all cases retains somewhat over one half per 

 cent, of alkalies, the potash predominating over the soda. 



It is to be noticed that in this Rocky Mountain region the 

 crystalline strata are more ancient than the Potsdam. The 

 same thing has been shown with regard to the quartzites 

 and talco-quartzose schists of Sauk and of Dodge counties, 

 Wisconsin. The former of these liad been referred by Hall to 

 the Huronian, but by most other geologists both have been 

 resrarded as altered Potsdam or St. Peter's sandstone. Pro- 

 fessor Roland Irving has pointed out that these rocks are in 

 both localities unconformably overlaid by the Potsdam sand- 

 stone, and thus confirms the view of Professor Hall. 



In the east, Mr. Ford has studied farther the Lower Cam- 

 brian rocks of Troy, New York, which, either by a dislocation, 

 or by an overturned and denuded fold, are made to directly 

 overlie, in apparent conforjnity, the Cambro-Silurian strata 

 (Utica and Hudson River slates) ; the whole of the strata dip- 

 ping to the eastward. These older, though overlying rocks, 

 contain forms belonoino- to the fauna which Mr. Billins^s has 

 named Lower Potsdam, including trilobites of the genera 3Ii- 

 cj'odiscus, Conocej^halites, Olenellus^ and Agnostiis. This fauna 

 is distinct from that of the true Potsdam of New York and 

 Wisconsin, as well as from the Menevian of IMassachusetts, 

 New Brunswick, and Newfoundland. Its relation to these is 

 yet to be determined, though it is closely allied to both. This 

 fauna is also found in Georgia, Vermont, and in the Strait 

 of Belle Isle, besides which a large extent of rocks on the 

 south side of the St. Lawrence below Quebec, formerly 

 mapped as the Quebec group, is now referred to the Lower 

 Potsdam. 



Professor Hall has again called attention to the distinct- 

 ness and importance of the Low^er Ilelderberg group of lime- 

 stones, which some have attempted to confound with the Ni- 

 agara, apparently because in the valley of the Ohio the two 

 series, reduced in volume, are in immediate contact. Hall 

 reiterates, what he had long since shown, that they are wide- 

 ly different in their fauna, and in central New York are sep- 

 arated by the great non-fossiliferous Onondaga or Salina for- 

 mation ; while to the eastward, Avhere this and even the un- 

 derlying Niagara is wanting, the Lower Helderberg appears 

 as a great fossiliferous limestone formation, which to the 



