MUSEUM OF COMrAKATIVE ZOOLOGY. 



321 



"Along the base of tlic Heldcrberg, where the Chnton, Niagara, and 

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

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

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



The same. Palsuontology of New York, Vol. III. 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 Silurians 

 are conformable on the northern front of the Ilelderberg Mountains 

 (p. 33, note). Farther on, he writes : "The Hudson Eivcr 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. GO); and again, "the unconformability of the Lower Heldcrberg 

 group upon the Hudson lliver 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. Leconte. 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 Gaspc, near Montreal, and at Becraft's 

 Mountain (210,* 241), The disturbance is thought not to extend 

 southwestward of New Jersey (217). 



J. G. Lindsey. A Study of the Eocks. Poughkeepsio Soc. Nat. Sci. 

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

 Eondout cement quarries, and regarding the junction of the Upper and 

 Lower Silurian rocks as unconformable. 



T. N. Dale. The Fault at Eondout. Amer. Jonrn. Sci., XVIIL, 

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

 Ilclderbergs, with a thin layer of Niagara (Encrinal) limestone at the 

 bottom, lying squarely across the tilted Hudson Eivcr strata. 



It would seem from this review that Mather and Eoircrs rejrardcd the 

 contact of the Upper and Lower Silurians as unconformable on both 

 sides of the Hudson ; Euunons iigures a conformable relation, and con- 

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

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



* Ilcm Becraft'a Mountain is wrongly said to be west of the Hudson, 

 VOL. vil. — i>o. 10, 21 



