OBISKANY FAUNA OF BECBAFT MOUNTAIN 7 



generalized section across the mountain from east to west is given on 

 plate 24, figure 6. The mountain is an outlier of principally early 

 Devonic strata about 2 miles in north and south extent, and these 

 rest unconformably, as shown by Mather, J. D. Dana and W. M. Davis, 

 on the upturned slates of the Hudson river formation. The most com- 

 plete and interesting portrayal of the structure of this area is that given 

 by Davis. 1 



Accompanying that paper is a sketch map of the mountain giving 

 the stratigraphic succession both in plane and in section. It is not my 

 purpose to enter into great detail with reference to the structure of this 

 area, but, to point certain of my observations, I have .taken the liberty 

 to represent on a somewhat different scale, the map given by Prof. 

 Davis, with some emendations of the contacts of the various formations 

 and the addition of two important divisions not noted by him. 



Over the northern portion the mountain is a simple, low syncline; 

 and toward the central part it shows a double syncline separated by a 

 very short fold, at which the layers of the Becraft limestone come to 

 the surface. Again, farther south the duplicate form of this syncline 

 is in a measure lost and the structure gradually becomes more compli- 

 cated, particularly at the southeast edge of the area. This was indi- 

 cated by Mather, who in one of his sections suggested that the rocks 

 at this point were overturned. Davis likewise left the structure here 

 in some doubt. The first strata resting on the upturned Hudson 

 river slates, 2 are those of the Tentaculite (Manlius 3 ) limestone, which 

 nearly inclose the entire area and on the northeast and southwest boun- 



'Amer. jour. sci. 1833. 3d ser. 26:381-89. 



2 At Mount Moreno, just south of the city of Hudson, these slates carry a 

 fauna with Coenograptus gracilis, Didymograptus Sagittarius, 

 D. tennis Hall and other graptolites identical with those of the Normanskill 

 shales at Kenwood and Glenmont, Albany county. The taxonomy of the great 

 mass of so-called Hudson river slates in the Albany region which has recently 

 been studied by Ruedeinann points to the early Trenton age of the Normanskill 

 shales and inferentially of these slates lying beneath the Manlius limestone of 

 Becraft mountain. 

 "See footnote, p. 8 



