Study of the New York Obelisk as a Decayed Boulder. 163 



the latter, between the arrival of the Obelisk at New York and the 

 autumn of 1885, had produced a widening of the pores in the sur- 

 face of the rock and incipient disintegration. This seems to me to 

 prove that the active absorption of water, in our rainy seasons, by 

 the minerals on the surface of the Obelisk, was the first and a con- 

 tinuous cause of decay. But there was, as plainly, a rending force, 

 apparently greater than that which can be attributed to expansion 

 b}^ hydration. 



(5). One result of decay, both in the granite of the quarry at 

 Syene and in that of the Obelisk during its 4f years exposure in 

 New York, consists in an increase of specific gravity, both in the 

 mineral matter and in the entire rock with all its interstices. This 

 is a further indication that the actual expansion by hydration, in 

 the decayed surface, just referred to above, must have been very 

 small, and that the rending force must be sought in some other 

 direction. 



The specific gravity of the granite of our Obelisk was determined 

 by Persifor Frazer in mass, including its cavities, at 2.6618; when 

 determined in grains of the size of a pea, at 2.7188; giving the 

 weight of one cubic foot of the rock at 166.1625 pounds avoirdupois. 

 According to G. W. Wigner, the specific gravity of the stone of the 

 London Obelisk was 2.682; absorbent power of the fresh stone, at 

 the rate of 5.4406 grams of water per square meter, and of the 

 weathered surface at a rate six times as great. 



There are only two other forces, to whose sudden application or 

 increased action the rapid exfoliation of the surface of the Obelisk 

 from 1881 to 1885 has ever been attributed. 



One of these is our climatic variation in temperature, with fre- 

 quent sudden changes within a single day, enhanced by the strong 

 heat of the sun. But I have already shown, from the even wider 

 ranges of temperature in the climate of Egypt, at a higher portion 

 of the scale, and from the observed results upon the sun-exposed 

 faces of all obelisks, that this supposed cause had little or nothing 

 to do with the surprisingly sudden disintegration which attacked 

 the Obelisk immediately after its arrival. 



It seems therefore established that we must attribute those visible 

 effects of decay entirely to the violent force which'was then exerted 

 upon the monolith, almost for the first time in all its history — that 

 of frost. The power exerted by the expansion of water in freez- 



