after sunset, and the rocks, strongly heated by day, are now
chilled perhaps even to the freezing point.

In the Sahara the thermometer has been known to fall 131 degrees
F. within a few hours. In the light air of the Pamir plateau in
central Asia a rise of 90 degrees F. has been recorded from seven
o'clock in the morning to one o'clock in the afternoon. On the
mountains of southwestern Texas there are frequently heard
crackling noises as the rocks of that arid region throw off scales
from a fraction of an inch to four inches in thickness, and loud
reports are made as huge bowlders split apart. Desert pebbles
weakened by long exposure to heat and cold have been shivered to
fine sharp-pointed fragments on being placed in sand heated to 180
degrees F. Beds half a foot thick, forming the floor of limestone
quarries in Wisconsin, have been known to buckle and arch and
break to fragments under the heat of the summer sun.

FROST. By this term is meant the freezing and thawing of water
contained in the pores and crevices of rocks. All rocks are more
or less porous and all contain more or less water in their pores.
Workers in stone call this "quarry water," and speak of a stone as
"green" before the quarry water has dried out. Water also seeps
along joints and bedding planes and gathers in all seams and
crevices. Water expands in freezing, ten cubic inches of water
freezing to about eleven cubic inches of ice. As water freezes in
the rifts and pores of rocks it expands with the irresistible
force illustrated in the freezing and breaking of water pipes in
winter. The first rift in the rock, perhaps too narrow to be seen,
is widened little by little by the wedges of successive frosts,
and finally the rock is broken into detached blocks, and these