23S T. W. Harris— Mount Bob or Mount Ida. 



characteristic Pentamerus galeatus were found. In the center 

 of the hill was a small patch of brownish-gray, shaly material 

 in which a few specimens of Spirifer macropleura and Stro- 

 jphomena rugosa were found ; apparently representing the 

 Delthyris or Catskill shaly limestone. The axis of the syn- 

 cline courses about N. 35° E., and its plunge is about 20°. To 

 the north it disappears under a heavy mass of drift. The dip 

 on the east side averages 25° ; on the west, 20°. 



The underlying strata, which appear at several points about 

 the base of the hill, are of a soft gray shale, without fossils, 

 but resembling in appearance the rocks which occur through- 

 out this part of the Hudson valley, and are known as the 

 Hudson River group. The shale has a well-marked cleavage, 

 which appears parallel to the stratification wherever the latter 

 can be distinctly made out. The strike varies from 1ST. 90° E., 

 at the northern end of the hill, to N. 5° W., at its southern 

 end, the dip being always very steep. The question of the 

 conformity of the limestone upon the shale was considered, 

 and a careful search made for their junction ; but this was 

 everywhere covered, the nearest approach of the two rocks 

 being about fifty feet, on the west side of the hill, at b, fig. 2. 

 Here the limestone had a strike of N. 30° E. and a dip of 22° ; 

 the shale, striking N. 8° E., and dipping 60° E. Near the 

 southern end of the hill, the strike of the shale, at c, fig. 2, 

 was N. 5° W. ; its dip, 45° E. At the northern end of the 

 hill (fig. 2, a) the strike of the shale, here very distinct, was 

 about N. 90° E. (varying about 60° from that of the limestone), 

 and its dip 80° S. : but the distance between them being some 

 four hundred feet, it is quite possible that the limestone, if 

 restored, might be found to come into parallelism with the 

 shale within that distance. But although the question of con- 

 formity cannot be fully settled from these outcrops, the gene- 

 ral lack of accordance between the strikes of the shales and of 

 the limestones, would seem here, as at Becraft's Mountain, 

 rather to favor unconformity, and to suggest that the lime- 

 stones were laid down upon the upturned and eroded edges of 

 the shales, probably over an area extending far from the out- 

 lier in all directions. Since then, erosion has removed most of 

 the limestone, this fragment owing its preservation perhaps to 

 its place in the bottom of a syncline deeper than the average, 

 and hence the last to be reached by the levelling forces; and 

 perhaps also in part to a local increase in the thickness of the 

 hard Tentaculite limestone, if we may take the thicknesses of 

 that member at ^Catskill (seventy feet) and Becraft's mountain 

 (forty to fifty feet) as average measures. 



Cambridge, Mass. 



