THE PROPERIVES (OF BULLDING STONES, ETC. 179 
in a dilapidated condition after it has been built a comparatively 
few years. When the snow melts in the spring the water sinks 
into the ground and issues through every crack and crevice in 
the wall. As it collects along these fissile planes it freezes and 
wedges apart the lamine of the rocks. 
Because the sedimentary rocks more frequently have parting 
planes than the igneous, they are as a class more apt to suffer 
from alternate freezing and thawing. On the other hand the 
sedimentary rocks are sometimes as free from parting planes as 
the igneous, and are accordingly in as little, or even less, danger 
from freezing. 
The openings known as caves, caverns, and cavities need not 
occupy our serious attention. Cavities occasionally occur in 
both sedimentary and igneous rocks used as building stone, but 
mainly in the former. They do not increase the danger from 
freezing, owing to the fact that they are seldom filled with water 
when near the surface. They weaken the rock slightly and often 
occasion a roughness of the face when they occur at the surface. 
The cavities are often partly filled with impurities, such as pyrite, 
which may injure the rock, through the readiness with which 
they decompose. 
From the foregoing we may conclude that an ordinarily well 
cemented sandstone, which is free from parting planes or strati- 
fication, and in which the pores are of greater than subcapillary 
size, is best suited to withstand alternate freezing and thawing 
when placed in the wall of a building; assuming that the original 
strength of the stone is sufficient for the position which it occu- 
pies in the wall. 
Mechanical abrasion —One of the most important agents of 
disintegration in nature is mechanical abrasion, but the rdle 
which it plays in the destruction of artificial structures is not 
nearly as important as that of certain other agents. 
Mechanical abrasion is accomplished mainly by wind, run- 
ning water, and shuffling feet working in conjunction with the 
other agents of disintegration. The beating of the rain against 
the stone wall may overcome the adhesion between the rock 
