Study of the New York Obelisk as a Decayed Boulder. 165 



logical point of view, already fully discussed in the paper referred 

 to, such an impervious metal film would serve as an efficient cover- 

 ing, to shed rain-water, sleet, and melting snow from the sloping 

 sides of the pyramidion and from the hollows of the deep intaglios 

 which cover the shaft below. To these hieroglyphs the Obelisk 

 owes its chief interest as a historical monument, and, unfortunately, 

 their cavities and projections, although still preserved, have suffered 

 the chief injury by the surface-decay. The regilding could be car- 

 ried out at small expense, and would be a most useful ally to the 

 waterproofing treatment. Nothing too much can be done by our 

 City authorities to secure both the preservation and proper decora- 

 tion of this unique Symbol of the Sun on American soil, and to 

 offset the deplorable neglect of our City, up to 1885, in its care of 

 this magnificent gift from a generous citizen, the late Mr. William 

 H. Yanderbilt. 



Finally, then, when we return to our rusty pebble and the 

 Egyptian boulder, what conclusions may we fairly draw as to the 

 conditions attending their decay ? 



A. The main agency, by far predominant over all others, in the 

 decay of these granite masses, has been aerated rain-water ; this has 

 been aided in Egypt by extreme and constant oscillations of tem- 

 perature. Their means of action have been two-fold. 



(1). Chemical, by absorption of water, together with oxygen, in 

 combination with part of the bases of the unlocked silicates, and 

 gradual removal of the rest in solution, producing irregular changes 

 of volume and proportionate increase of the mineral-clefts. 



(2). Mechanical, through expansion by the sun's heat and con- 

 traction by night-radiation, aided by artificial roasting in some 

 cases, tension and jar during transportation, further washing out 

 of soluble matters, and, in our climate, freezing : all producing 

 increase of the rock-pores. 



B. The rate of action of each process is approximately indicated 

 by some of the facts stated : — 



(1). Chemical action with a limited rainfall, efficiently aided by 

 oscillations of temperature, has tended, in the hot and comparatively 

 arid climate of Syene, to cause the disintegration and removal of a 

 layer, at least one centimeter in thickness, from the surface of the 

 granite-cliffs at Syene, during a period of five to six thousand years. 

 Forty-five centuries have been generally insufficient to produce any 

 visible external injury and exfoliation. 



