108 NEW YORK STATE MUSEUM 



one passes in the Hudson River plain over Snake Hill shales, fre- 

 quently covered by quaternary clays, until near the foot of the 

 hills marking the edge of the Greenwich plateau. Here there is 

 a belt of much broken and distorted Beekmantown limestone (Bald 

 Mountain limestone) and coarse conglomerate (Rysedorph Hill 

 conglomerate), and the hills themselves and the plateau behind 

 them consist of the Georgian rocks. The latter, although im- 

 mensely older, hold a higher level than the shales west of them. 

 The explanation of this remarkable fact is that the Georgian 

 has been overthrust upon the Ordovicic rocks. This is most 

 clearly shown at Bald mountain where the former quarrying of 

 the limestone at the foot of the mountain has opened a splendid 

 section (see plate of sections). 



Bald mountain is, geologically speaking, an historical mountain. 

 It has played a conspicuous role in the Taconic controversy and 

 it is therefore proper in this place briefly to review its history. 

 Emmons was the first to direct attention to the Bald Mountain 

 section as being an interesting example in the relative position of 

 the "Taconic" and " Champlainic " (Ordovicic) rocks, that is, as 

 showing the Champlainic rocks as resting unconformably on the 

 Taconic rocks. He gave (1846, page 89) a section through Bald 

 mountain showing the " calcif erous sandstone " extending from 

 the top of the mountain to the western foot where it is underlain 

 by the " Taconic " black slates, which also appear on the eastern 

 foot of the mountain. The two trilobites Atops trilineatus 

 and Elliptocephala asaphoides, which demonstrated 

 the actual presence of rocks older than Potsdam in the slate belt, 

 were also cited as coming from the strata " near Bald mountain " ^ 

 {op. cit., page 63). 



Walcott {op. cit., page 317) has shown that the complex struc- 

 tural relations of Bald mountain are entirely different from what 

 Emmons supposed them to be. He found Georgian fossils in the 

 limestones, forming the summit of the mountain, and states : 

 " Doctor Emmons identified this mass of strata, d, with the cal- 

 ciferous sandrock on lithologic characters, overlooking the fact 

 that a similar rock might occur in his Taconic series." Walcott 

 also recognized the overthrust at Bald mountain. He says : " The 

 section of Bald mountain proves that the strata of the ' Upper 



1 Walcott {op. cit., p. 326) cites them as coming from the black slate 2 miles 

 north of Bald mountain, where Doctor Fitch found them. 



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