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 
