OCCLUSION OF IGNEOUS ROCK 395 



southern end of the diorite outcrop is also probably underlaid 

 by dolomitic limestone. 



Map and Cross-section. — While the general strike of the 

 region, N. 45 to 50° E., prevails over this tract, an eastward 

 bending toward the northern end to N. 61° E., may be due to 

 the influence of the enclosed intrusive mass. It may be here 

 noted that the other great bed of hornblende-schist and serpen- 

 tine at West 59th Street, whose thickness approximated 30 feet,' 

 presented a violent change of course from northeast around to 

 north. "^ Apparently in each case the disturbance may have 

 been a remnant of ancient unconformity, the surrounding 

 gneisses having been later crushed and adjusted into partial 

 accordance with the intruded mass. Changes in the direction 

 and amount of the dip, plotted on the accompanying sketch- 

 map (Plate V) indicate the emergence of a series of sharp folds, 

 the anticlines being partly overturned to the eastward and asym- 

 metrical, their eastern legs being very nearly vertical. Two of 

 these at one point are shown in the cross-section (Plate VI), ap- 

 proximately on the line of 216th Street. In addition, the vari- 

 ations in dip, often nearly vertical, along the eastern margin of 

 the granitoid or pegmatitic micaceous gneiss, which constitutes 

 the crest and mass of the hill, may signify the survival there of 

 another sharp fold. This may imply that the mass composing 

 the w^estern slope is made up of a series of sharply compressed 

 folds, obliterated during pegmatization, with assumption of a 

 new foliation and system of jointage. 



Near the center of the hornblendic tract these folds have 

 produced an expansion of the outcrop of diorite-schist to a 

 breadth of more than 200 feet, from w^hich an exaggerated idea 

 of its extent and thickness might be inferred. The exhibition 

 of structure denoting vicinity to the bottom of a syncline (Plate 

 VI), at about 216th to 217th Street, and the low dips at the top 

 of an anticline, near the northern disappearance of the outcrop, 



^ H. Credner, " Die Gliederung der eozoischen (vorsilurischen) Forniations- 

 gruppe Nord-Amerikas," Habilitationsschrift, Halle, 1869, 19. 

 ^Julien, loc. cit., Map, Fig. 9, p. 488. 



