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



(2). Mechanical action, predominantly that of frost, has worked 

 in the climate of New York at a vastly more rapid rate. On the 

 surface of the Obelisk, already scarred and weakened by fire, it 

 completely loosened a shell of about 0.13 mm. in thickness in 4| 

 years, equivalent to 1 centimeter in 70 years — or more nearly 1 

 centimeter in 50 years, when we allow for the decayed and parti- 

 ally loosened material which has not been removed from the surface 

 of the monument. But although this rate far exceeds that of the 

 estimate of Dr. Barnard (1 cm. in 6000 years) there is no evidence 

 that it must be continuous ; the chief exfoliation has been probably 

 already effected in the weakened, thin outer layer of stone ; the 

 main stone below is practically sound. 



It appears then that an ancient column of granite like this, while 

 unfitted to mark the flying hours on a Roman dial, may yet serve 

 us as a true gnomon to record some phases of rock-decay at inter- 

 vals in geological time. 



