ISJo. 104.] 203 



occasion ; and so on successively with alternate freezing and thawing 

 until an unsightly crevice is produced, which constantly widens and 

 encroaches more or less on the adjacent parts till the stOne is de- 

 stroyed. 



This condition occurs in the gray or light-colored freestones, as 

 well as in the brown ones; but in the brown freestone or sandstone, 

 there is a further cause of destruction. The coloring matter, which 

 is also in part the cementing matter of the grains of sand, is ferru- 

 ginous, the siliceous grains are covered with peroxide of iron, and 

 this substance is intimately combined with the argillaceous matter of 

 the mass which cements the particles. Experience has everywhere 

 proved that the brown sandstones or freestones are not durable 

 stones ; their destructibility is not only due to the presence of 

 argillaceous matter, but to the oxide of iron ; for the gray or neutral- 

 tinted stones, of the same composition otherwise, are much more 

 durable. 



As an evidence of the rapid decomposition of the red or brown 

 sandstone when the siliceous element is deficient, we may sometimes 

 find a large area, which, when broken up, decomposes so rapidly 

 that it becomes in a few years an arable soil. The same is essen- 

 tially true in some parts of the Medina sandstone. In order to 

 demonstrate this fact, it is only necessary to examine any building 

 of brown stone which has been erected for a period of twenty -five 

 years. The State Library building is an example in point. The 

 Capitol and the Albany Academy have been longer erected, and 

 were originally of better material than the Library building. The 

 basement of the old City Hall in New York is an example of the 

 same kind, where the brown stone, from its inherent destructibility 

 and from the presence of clay seams, presents a dilapidated appear- 

 ance ; and other examples might be mentioned. In Europe the 

 same condition exists, and many old buildings of the red or brown 

 sandstone are falling in ruins. 



In the lighter-colored sandstones, we have mainly to guard 

 against clay seams and too large a proportion of argillaceous mixture 

 in the mass. Beyond this, the presence of iron pyrites is to be 

 looked for. This mineral is present in so many rocks of this 

 character, especially those with a bluish or greenish olive tint, that 

 it is to be suspected in all such stones. It should be remarked, 

 moreover, that iron pyrites (sulphuret of iron), when in visible grains, 

 nodules or crystals, is not so dangerous or destructive to the rock as 

 when disseminated in fine or im_.perceptible srains through the entire 

 mass. This mineral, however, may be so disseminated and not 

 prove entirely destructive, since in some stones it decomposes from 

 the first exposure to the weather, staining the exterior of a rusty 

 hue, and thus continuing to exude as an oxide of iron so long as any 

 of it is reached by the moisture of the atmosphere ; at the same 

 time the free sulphuric acid unites with the lime or magnesia, if 

 either be present, or to some extent with the alumina in the absence 

 of the other substances j and this chemical change may sometimes 



