BUILDING AND ORNAMENTAL STONES. 339 



expose fresh surfaces to be acted upon. Certain of the ferruginous 

 cements are likewise susceptible to the influence of the acidulated rains, 

 though the anhydrous oxides occurring in the Potsdam stones are, ac- 

 cording to Julien, less soluble than are the hydrated forms occurring in 

 those of Triassic age.* The feldspars of granites and other rocks are 

 also susceptible to the same influence, though naturally in a much less 

 degree. The acidulated rains aided by the disintegration produced by 

 temperature changes may in time partially remove, in the form of car- 

 bonate, the alkalies — potash and soda— and the rock slowly disinte- 

 grates into sand and clay. The feldspars of the gneiss, used so ex- 

 tremely in years past in and about Philadelphia, are said to have proved 

 peculiarly liable to this change, and it has been found necessary in many 

 instances to paint some of the older structures formed from it to avoid 

 serious disintegration. 



(3) INDURATION OF STONE ON EXPOSURE. 



The changes produced by weathering are not in all cases those of de- 

 composition. All stones, and especially the limestones and sandstones, 

 undergo at first a process of hardening on being removed from the 

 quarry or when exposed in the quarry bed, as will be noted further 

 on. This hardening is explained by ISewberry and others on the sup- 

 position that the water with which the stones are permeated, holds in 

 solution, or at least in suspension, a small amount of siliceous, calca- 

 reous, ferruginous or clayey matter. On exposure to the atmosphere 

 this quarry water, as it is technically called, is drawn by capillarity to 

 the surface of the block and evaporated. The dissolved or suspended 

 material is then deposited, and serves as an additional cementing con- 

 stituent to bind the grains more closely together. It is obvious that 

 the amount of induration must in most cases be quite small, and lim- 

 ited to but a thin outer crust on each block ; also that when this crust 

 has once formed it can, if removed, never be replaced since the stone 

 in the walls of a building is cut off from further supply of quarry water, 

 and as a matter of course, after whatever quantity contained within its 

 own mass has come to the surface and evaporated, no further hardening 

 by this means can take place. This induration sometimes takes place 

 in a peculiarly rapid and interesting manner. Dr. Wadsworth, in writ- 

 ing on some Potsdam and St. Peter's sandstones near Mazo Manie, 

 Wis.,t states that those portions of the stone which are exposed to at- 

 mospheric influences have become by induration converted into com- 

 pact quartzites, while the protected portions still retain their porous 

 and friable nature. So rapidly does this change take place that an 

 exposure of but a few months is sufficient to produce very marked re- 

 sults on a freshly broken surface. 



It is on this account that the practice of setting rough stone in a 



* Julien, Rep. Tenth Census, Vol. x, p. 77G. 



t Proc. Bos. Soc. Nat, Hist., Vol. xxn, 1883, p. 202. 



