MUSEUM OF COMPAEATIVE ZOOLOGY. 321 



" Along the base of the Helderberg, where the Clinton, Niagara, and 

 Onondaga salt groups are very thin, the Oneida conglomerate is absent, 

 and the shales and sandstones of the Hudson River group rise to within a 

 few feet of the Tentaculite limestone or Waterlime group." (p. 1, note.) 



The same. Palaeontology of New York, Vol. 111. 1859. The author 

 repeats a doubt previously expressed as to the truth of the unconformity 

 at Becraft's Mountain, and states that the Upper and Lower Siluriana 

 are conformable on the northern front of the Helderberg ^lountains 

 (p. 33, note). Farther on, he writes : " The Hudson River group, which 

 constitutes a few feet of their [Catskill Mountains] elevation at the base, 

 is disturbed, and the succeeding beds lie upon this unconformably " 

 (p. 69); and again, "the unconformability of the Lower Helderberg 

 group upon the Hudson River group " shows that the subsidence of the 

 old sea bottom was periodical {p. 70 ; see also p. 88). 



These are the older observations on the subject. The following ref- 

 erences show the recent work, as well as the lack of agreement on the 

 question in the two text-books in more general use. 



J. Lecoute. Elements of Geology. New York, 1878. " In the United 

 States, the rocks of the whole [Paleozoic] system are conformable." 

 (p. 277.) 



J. D. Dana. Manual of Geology. New York, 1880. The making of 

 the Green Mountains came at the end of the Lower Silurian (211, 212) ; 

 the disturbance extended to the Hudson (214). Localities of uncon- 

 formity mentioned are near Gaspe, near Montreal, and at Becraft's 

 Mountain (216,* 241). The distm-bance is thought not to extend 

 southwestward of New Jersey (217). 



J. G. Lindsey. A Study of the Rocks. Poughkeepsie Soc. Nat. Sol. 

 Proc, II., 1879, 44-48, giving a careful description of the rocks at the 

 Rondout cement quaiTies, and regarding the junction of the Upper and 

 Lower Silurian rocks as unconformable. 



T. N. Dale. The Fault at Rondout. Amer. Joum. Sci., XVIIL, 

 1879, 293-295, from which figure 15 is here copied, showing the Lower 

 Helderbergs, with a thin layer of Niagara (Encrinal) limestoue at the 

 bottom, lying squai-ely across the tilted Hudson River strata. 



It would seem from this review that Mather and Rogers regarded the 

 contact of the Upper and Lower Silurians as unconformable on both 

 sides of the Hudson ; Emmons figures a conformable relation, and con- 

 siders the apparent unconformity at Rondout the result of a fault ; Hall 

 at first admitted the general unconformity, but later doubts it even for 



* Here Becraft's Mountain is wrongly said to be west of the Hudson. 

 VOL. vii. — KG. 10. 21 



