1875.] 191 [Hitchcock. 



present day, and which had been discussed at a previous 

 meeting. He exhibited specimens of Ilyanassa obsoleta 

 from shell heaps at Marblehead, which, in the drawn-out 

 spire and thickened shell, showed important differences from 

 the specimens of the species now living in the same locality. 



Prof. C. H. Hitchcock briefly stated some conclusions he 

 had recently reached concerning the stratigraphical structure 

 of the Cambrian and Cambro-Silurian rocks of Western 

 Vermont. 



His observations led him to believe that Emmons understood the 

 stratigraphical relations of these rocks (many of them called Taconic 

 by him) better than most of his contemporaries, while the recent 

 discoveries of fossils do not confirm the disposition of the great mass 

 of the Taconic system as Cambrian. 



Emmons believed these rocks were deposited successively against 

 the western base of the Green Mountains; first the granular quartz, 

 then the Stockbridge limestone, and lastly, the various slates which 

 were capped by the black slates holding Olenellus, which is really 

 the oldest member of the series. Prof. Hitchcock suggested as a 

 better theory of structure, that sediments were formed contempora- 

 neously, both upon the Green Mountains and the Adirondack side 

 of the valley, thus making the granular quartz on the east side of 

 the valley of the same age with the Potsdam sandstone at White- 

 hall, N. Y., and elsewhere west of Lake Champlain. Next, the 

 Calciferous sandstone, Levis, Chazy and Trenton limestones, were 

 deposited entirely across the valley, and by means of their fossils are 

 now identified adjacent to both the quartz rock and the typical Pots- 

 dam sandstone. Thirdly, the limestones are succeeded by slates. 

 This theory of original deposition differs from that of Emmons, in 

 supposing that sedimentation was being effected both on the Green 

 Mountain and Adirondack borders, instead of on the former only. 



The origin of the present arrangement of the strata, with a usual 

 easterly dip and numerous faults, may be understood by recalling the 

 character of the folds in Ferrisburg, Monkton and Starksboro. The 

 Potsdam sandstone occupies most of the country along this section, 

 and there are at least six folds between Lake Champlain and the 

 Green Mountains. First, the Chazy and other limestones in the 



