Study of the Nexo York Obelisk as a Decayed Boulder. 101 



weathered or fallen out and formed the well-known rift or notch' at 

 the east base, partly filled and pointed with cement, at the time of 

 the treatment of the Obelisk in 1885. 



There is an interesting; correspondence, in both constitution and 

 origin, between the rocks of New York Island and those of Syene. 

 The so-called " Gray wacke Knoll," on which the Obelisk now 

 stands, consists of biotitic hornblende-schist and gneiss, closely 

 resembling the black seams in the monolith. This mass is crossed 

 by a vein of coarse endogenous granite, very similar in places to 

 that of the Obelisk itself, which is now covered by the western 

 steps leading up to the platform ; some branching seams of this 

 granite still project on the sides of the steps. On account of this 

 resemblance, except in the brighter red color and porphyritic char- 

 acter of the Obelisk-granite, a box of fragments of rubbish from this 

 vein was kept at hand by the workmen, at the time of the water- 

 proofing of the monument in 1885, to satisfy the constant demands 

 of visitors from all parts of the country for specimens from the 

 monument, and admirably answered the purpose to the gratifica- 

 tion of both parties. 



It would appear that the strongly marked bedding, apparent in 

 photographic views of the old quarries at Assouan, and in con- 

 formity with which all the obelisks wei'e hewn, is not, at least in 

 all cases, the true plane of original stratification. This bedding 

 plane is shown in the gneissoid structure of our Obelisk and now 

 stands upright in the shaft. But, to the geologist's eye, the New 

 York Obelisk is mereh" a long block of biotitic, porphyritic granitoid 

 gneiss, in part hornblendic, crossed by seams and lenticular nodules 

 of black hornblende-schist, whose lamination (probably signifying 

 the true original bedding) now happens to be set up, so to speak, 

 with a strike of W.N.W. to E.S.E., and a dip of 40° to N.N.E. 



3. Entasis of E.S.E. face of the New York Obelisk. 



While here discussing the locality and original source of the 

 material of Egyptian obelisks, we may refer to one feature of the 

 New York monolith to which my attention was first called by 

 Prof. R. 0. Doremus, a slight curvature, longitudinally convex, of 

 its present E.S.E. face. On farther examination, there appeared to 

 me, also, a very slight lateral convex curvature of the same face, 



' Gorringe, op. cit., 12. 



