98 Study of the New York Obelisk as a Decayed Boulder. 



greatly in lithological constitution. Commonly it consists of bright 

 red to yellowish red microcline in large twins; white oligoclase, 

 sometimes yellowish or greenish ; smoky and gray quartz ; black 

 biotite, sometimes brown or green ; the last often replaced in part 

 or altogether by black amphibole. Less commonly occur yellow 

 mica, pyrite, magnetite, and dark brown garnet. Hematite in 

 hexagonal or rhombic reddish plates, yellowish red titanite, color- 

 less apatite, zircon, viridite, and yellowish green needles of pistazite 

 have also been detected. Newbold also reports:^ "Schorl, black 

 and green, and actinolite are minerals occasionally found in the 

 granite of Upper Egypt, as well as the chrysoberyl." 



In the quartz, Stelzner also distinguishes capillary black needles, 

 which I have recognized as rutile; and in its larger grains, cloud- 

 like zones of fluid cavities, in the smaller of which the bubbles 

 show invariably more or less motion. To this I can add, from 

 examination of my own thin sections of rock from the Obelisk, that 

 the fluid contents of these cavities consist sometimes of brine, some- 

 times of liquid carbon dioxide. Delesse attributes its smoky tint 

 to the presence of a very small quantity of organic matter. 



As to the proportion of the main constituents, the following per- 

 centage results have been reported: — 



Mica ..... 

 Quartz .... 



Microcline .... 

 Oligoclase .... 



100 100 



In my examination of the four sides of the Obelisk in 1890, while 

 hanging in a chair from its summit during several days, I recog- 

 nized, in addition to the common constituents already named, the 

 occasional presence of magnetite, and, on the upper part of the N.N.E. 

 face, very rare particles of pyrite, giving rise to slight ochreous 

 rings of decomposition. 



2. Distribution and condition of minerals on the surface 



of the Obelisk. 



In examining the W.N. W. face of the shaft, black mica was found 

 to be specially abundant, in bright scales in large part inclined about 



1 Newbold, loc. cit., 340. 2 Delesse, loo. cit., 489. 



3 G. W. Wiguer, loc. cit. 



