DESTRUCTION or ROCKS. 2?7 



lacks which thus cause the richest works of man to crumble si- 

 lently away. 



It is painful," says Philips, "to mark the injuries effected by 

 a few centuries on the richly sculptured arches of the Romans, 

 the graceful mouldings of the early English architects, and the 

 rich foliage of the decorated and later Gothic styles. The chang- 

 ing temperature and moisture of the air communicated to tho 

 slowly conducting stone, especially on the western and southern 

 fronts of buildings, bursts the parts near the surface into powder, 

 'or, by introducing a new arrangement of the particles, separates 

 the external from the internal parts, and causes the exfoliation or 

 desquamation, as Mac Culloch calls it, of whole sheets of stone 

 parallel to the ornamental work of the mason. From these at- 

 tacks no shelter can wholly protect; the parts of a building which 

 are below a ledge often decay the first; oiling and painting will 

 only retard the destruction; and stones which resist all watery 

 agency, and refuse to burst with changes of temperature, are 

 secretlv eaten away by the chemical forces of carbonic acid and 

 other atmospheric influences. What is thought to be more dura- 

 ble than granite ? Yet this rock is rapidly consumed by the 

 decomposition of its felspar, effected by carbonic acid gas; a pro- 

 cess which is sometimes conspicuous even in Britain, but is usu- 

 ally performed in Auvergne, where carbonic acid gas issues plen- 

 tifully from the volcanic regions." 



The most durable rocks appear to be the limestones of the 

 Silurian formation, but some of the sandstones of the more re- 

 cent formations are exceedingly perishable. This may be seen 

 upon examining the tombstones of red sandstone, which were 

 formerly used very extensively in New England, their inscriptions 

 will generally be found illegible. The durability of sandstones 

 depends much upon the nature of the cement which binds the 

 particles together, if this be calcareous they are not so durable as 

 if silicious, and if the stones contain iron, they are generally 

 highly perishable. Such stones which, may always be detected 

 from their iron -brown, or rusty appearance, should be rejected 

 for building stone. 

 M 



