360 



DURABILITY OF ROCKS. 



feldspar and but little iron, are as durable rocks as 

 this country anywhere affords. 



All rocks which readily absorb moisture, such as 

 our red sandstone ox freestone, are very perishable. 

 This may be seen by examining the tombstones of 

 this material, which were formerly employed to 

 considerable extent in this country ; their inscrip- 

 tions will generally be found illegible. In many 

 buildings in this city (New- York), the surface of 

 walls constructed with it is often rough and un- 

 seemly, owing to its absorbing moisture, which, on 

 freezing, peels it off in flakes. There is, however, a 

 great difference in freestone in this respect, some of 

 them not possessing great porosity, and, of course, 

 are more durable. Regard, however, should be 

 paid to the several beds of which a rock is com- 

 posed. It does not follow that, because one portion 

 of a rock is perishable, the whole of it is so. For 

 example, Philips states that Roman sculptures re- 

 main at Bath and York, executed in oolite, magne* 

 sian limestone, and millstone grit, which yet retain 

 all their characteristic perfection ; while other rocks 

 of the same species have perished in churches and 

 houses in less than 100 years. Many other similar 

 facts could be given to show that the different beds 

 of a rock are of very unequal value. As certain 

 trees will bear the ocean air and others not, so with 

 stone ; it is not equally durable in all situations, but 

 yields variously and unequally to carbonic acid, 

 smoke, dampness, and salt vapours. As a general 

 rule, we think it will hold good, that the durability 

 of building-stone may, in a great measure, be esti- 

 mated by the quantity of water which it absorbs ; 

 that which imbibes the most being the most perish- 

 able, and vice versa. The durability of sandstones 

 depends much, however, on the cement with which 

 the small particles are bound together. If it be 

 silicious, the stone will prove more durable than if 

 it be calcareous ; in which case it is readily acted 



