PTE ROPE RILES OME ULEDING STONES, £ TG. 173 
The external forces of destruction may be conveniently con- 
sidered in two classes: (1) those that produce changes through 
mechanical disintegration and (2) those that produce changes 
through chemical decomposition. In the case of disintegration the 
adhesion between the particles or the cohesion of the particles 
themselves is overcome, and the rock ultimately crumbles into 
sand or powder. In the case of chemical changes the identity 
of the mineral particles themselves is destroyed, by the minerals 
being broken up into other compounds. 
The following is a general classification of the agents of 
mechanical disintegration and chemical decomposition: 
I. AGENTS OF MECHANICAL DISINTEGRATION 
A. TEMPERATURE CHANGES. 
1. Unequal expansion and contraction of the rock and its mineral 
constituents. 
2. Expansion occasioned by the alternate freezing and thawing of 
the interstitial water. 
B. MercuanicaL ABRASION. 
1. Water. 
2. Wind. 
Bo, SALE 
GROWING ORGANISMS. 
CARELESS METHODS OF WORKING AND HANDLING STONE. 
oie 
Il. AGENTS OF CHEMICAL DECOMPOSITION. 
WATER-SOLVENT ACTION. 
CARBON DIOXIDE. 
SULPHUROUS ACIDS. 
. ORGANIC ACIDS. 
Temperature changes.—Injuries to a stone through changes in 
temperature are occasioned in two ways: (1) By the unequal 
expansion and contraction of the rock and its mineral con- 
stituents, and (2) through expansion due to the alternate freez- 
ing and thawing of the interstitial water. 
GaAwW LS 
