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



more inclined to powder than on the north side. At the bottom of 

 the shaft occurs the great seam of hornblende, of which the cleft has 

 now been partly filled with cement. 



On the S.S.W. side of the shaft, at the top, the feldspar and mica 

 appear bright on all the fresh surfaces. About a third of the way 

 down, near the second row of cartouches, the red microcline, quartz, 

 and mica continue to be wonderfully bright and glittering ; the 

 feldspar crystals sometimes 3 inches long by | of an inch wide, and 

 the quartz in occasional flakes, 3 to 4 inches long. A small lens of 

 hornblende-gneiss, 2 inches long, was seen just below the pyramid- 

 ion, but none further down. About 22 feet above the bottom of the 

 shaft, the grains of quartz and feldspar are often bright, and appa- 

 rently with as few cracks as in any fresh granite; the feldspar crys- 

 tals are salmon-colored to pink, generally 1^ inches long by ;^ to ^ 

 inch wide, and some show dull lustre. The white grains of oligo- 

 clase are here abundant, dead-white and covered with snow-white 

 films (calcium carbonate?), forming irregular dull spots, ^ to | inch 

 in length. Many little flakes of black hornblende here occur, ap- 

 parently as numerous as those of black mica, and often surrounded 

 by ochreous particles and spots. The scales of black mica are shin- 

 ing and flat, and never show curling. Along the bottom of the 

 lowest cartouche, in the east column, near the bottom of the shaft, 

 the black streaks consist of flakes of black hornblende. A crystal of 

 microcline was noticed below, with pale altered edge. 



In regard to the distribution of the biotite and hornblende on the 

 four faces of the Obelisk, I found that it varies greath^, biotite in 

 general largely replacing the hornblende. Where the latter occurs, 

 it may be alone and scattered in grains, or intermixed and closely 

 interpenetrated with biotite, or concentrated in large masses, often 

 lenticular in outline, or thinning out at one or both ends into wedge- 

 like seams. In these masses, the plates and bunches of hornblende, 

 as well as of any biotite intermixed, are arranged in nearly parallel 

 planes ; so that, in fact, they present all the features of intermixed 

 masses of hornblende-schist, more or less biotitic. Still further, the 

 planes of these schist-enclosures lie very nearly parallel, so that this 

 obelisk-mass presents to us the last stage of a transition of horn- 

 blende-schist into a gneissoid hornblendic or biotitic granite. The 

 most extensive of these enclosures of hornblende-schist is that near 

 the base of the shaft which forms a narrow black seam running up 

 the W.N. W. face, and, on the E.S.E. face, has in olden time partly 



