TLE PROPER IME S VOL ROLE DING SRONES, ATC. 175 
The igneous rocks on account of their heterogeneous min- 
eralogical composition, interlocking character of the mineral 
individuals, and difference in size, are more liable to injury from 
the diurnal changes of temperature than are the unaltered sedi- 
mentaries. 
Investigation shows that, in arid regions, very great work is 
accomplished simply through expansion and contraction due to 
diurnal temperature changes. Merrill, in his ‘Rock Weather- 
ing,” cites an instance in Montana where he found ‘‘along the 
slopes and valley bottoms numerous fresh, concave, and convex 
chips of andesitic rock, which were so abundant and widespread 
as to be accounted for only by the diurnal temperature variations. 
During the day the rocks became so highly heated as to become 
uncomfortable to the touch, while at night the temperature fell 
nearly to the freezing point.’’’ 
Hire Of Lock surfaces in’ Africa to rise as high as 137, FE. in the 
day, and cool off so rapidly by night as to split off rocks 
Livingstone reports the tempera- 
weighing as much as 200 pounds. The expansive force of heat 
is well shown in many of the limestone quarries in Wisconsin, 
where beds from five to six inches in thickness are for the first 
time exposed to the heat of a summer’s sun. These thin beds 
become heated throughout their entire thickness and arch up 
on the floor of the quarry, generally breaking and completely 
destroying the stone. 
Many buildings show the effect of weathering on the side 
exposed to the direct rays of the sun, while the sheltered side 
remains uninjured. The only rational explanation for this is 
found in the diurnal temperature changes. Ordinarily the move- 
ments due to temperature changes are necessarily small, but after 
centuries of time they must invariably result in the weakening 
and final disintegration of the stone. 
Expansion occasioned by the alternate freezing and thawing of the 
included water.—The effects of diurnal temperature changes as 
described above, are smali when compared with the action of 
continued freezing and thawing on a rock saturated with water. 
™GEORGE P. MERRILL: Rocks, Rock Weathering, and Soils, p. 181. 
