340 REPORT ON NATIONAL MUSEUM, 1886. 



wall, and leaving them to be carved when the structure is completed, 

 is stongly condemned by some,* as in so doing the hard outer crust 

 that began to form as soon as the stone was exposed to evaporation is 

 entirely removed, and the delicate carving disintegrates much more 

 rapidly than otherwise would have been the case. The carving, it is 

 argued, should be done at once, while the quarry water is still present, 

 and the crust then forms upon its surface, and it is thus better able to 

 resist atmospheric action. The rescouring and honing of buildings and 

 works of art is strongly objected to on similar grounds.! 



(7) WEATHERING PROPERTIES OF STONES OF VARIOUS KINDS. 



We will now consider the effects of the various agencies just enumer- 

 ated upon the different classes of rocks in common use for building 

 materials. 



Granites are liable to disintegration chielly from the constant expan- 

 sion and contraction caused by natural temperatures. The chemical 

 changes to which they are subject, such as the kaolinizatiou of the 

 feldspars or rusting of the micas, being as a rule scarcely noticeable 

 in the walls of a building, while they are so compact as to be practically 

 non-absorbent, and hence not liable to injury by freezing alone. The 

 same may be said respecting the diabases, melaphyrs, and basalts when 

 not particularly rich in magnetite or secondary calcite. Dr. Hague, 

 in describing the decay of the granite obelisk in Central Park, New 

 York, says: ,u In my opinion the process of disintegration has been an 

 extremely slow one, caused by a constant expansion and contraction 

 of the constitutent minerals near the surface, due to diurnal variations 

 of temperature. In a climate like that of New York, where these diurnal 

 changes are freciuently excessive at all times of the year, the tension 

 between the minerals would naturally tend to a mechanical disinte- 

 gration of the rock. Granite being a poor conductor of heat, the effect 

 of these changes would be felt only at short distances below the surface, 

 causing in time minute fractures and fissures along lines of weakness. 

 Into these openings percolating waters, upon freezing, would rapidly 

 complete the work of destruction." £ 



The decay of the obelisk since it reached New York, then, has been 

 simply mechanical and not chemical. The same has been found true by 

 Professor Julien of certain granites used for building in New York City.§ 



Helmerson explains the rapid disintegration of the Alexander column 

 in St. Petersburg, Russia, on the grounds that it contains many large 

 crystals of a triclinic feldspar, which when subjected to the extreme 

 temperatures of Russian climate expand and contract unequally in the 

 direction of their three crystallographic axes and hence cause the 



*Le Due, Story of a House, p. 143. 



tSee Chateau, under "Inconvenience du grattago a vif," p. 353. 



t Science, December 11, 1885, p. 511. 



§E. g., the old " Tombs" building on Center street. 



