in the Wappinger Valley Limestone. 127 



but the minute interstices are occupied by calcareous material, 

 so that the rock effervesces a little under acids. 



In many places, again, the material takes the form of brec- 

 ciated conglomerate ; this is well shown in the western Potsdam 

 hill at the northern extremity of the tract under consideration 

 (Hill B), and its extension south into the next field (Hill D) ; 

 also in the western marginal Potsdam ridge just south of T. A. 

 Hinkle's house in the next field. 



The southeastern extremity or fork of the main fossiliferous 

 hill (A) is a solid mass of peculiar oolyte, made up of spherules 

 which are simply aggregations of rhomboidal calcite crystals 

 imbedded in the interstitial mass of quartzyte or calcareous 

 quartzyte. Ihave found pebbles of this peculiar oolyte in the 

 conglomerate above mentioned in the ridges of the western 

 margin, suggesting a later date of deposition for the latter. In 

 many places these Potsdam rocks exhibit very distinctly oblique 

 lamination. 



The strike and the dip are quite variable. The most general 

 strike of these Primordial strata appears to be about N. 21 E. 

 (true), and the most general dip about 55° easterly. Some fur- 

 ther statements as to local variations will be given subsequently. 



In order to determine the true stratigraphic relations of these 

 Potsdam and related strata, I have made a special detailed 

 examination of a district covering about a mile and a half 

 square in. the vicinity of this locality, and embracing about a 

 mile and a quarter of longitudinal extension in the direction of 

 the strike. This district is mapped in Plate VI. 



This Potsdam rock is one of the component members of the 

 most western, and by far the broadest of three parallel belts of 

 the Wappinger Valley limestones, which, in this vicinity, rise 

 between the Hudson River shales. In general, all these lime- 

 stones and shales lie in a series of abraded folds, having usually 

 a conformable strike of from N. 20° E. to K 30° E. These 

 folds are closely compressed, and pushed over to the west, so 

 that the earlier limestones usually overlie by inversion the later 

 shales lying to the west of them. 



The particular belt of limestone which stretches southwest 

 from the Driving Park is about 3500 feet in width at its 

 northern extremity, and about 6000 feet wide at the southern 

 portion of the limited district now under consideration. This 

 southern transverse line of 6-00 feet passes through the middle 

 of the estate of TV. S. Johnston on the Albany Post Road. 



This belt terminates toward the north, quite abruptly, along 

 an almost straight line which cuts it apparently at a right angle 

 to the strike. This line runs along the southern margin of the 

 Driving Park, continues east through the fields parallel to 

 Hooker avenue till it meets the Hudson River shale in the 



