12 SOILS AND FERTILIZERS 



10. Action of heat and cold. — Rocks, as we have seen, 

 are mixtures of different minerals. These minerals have 

 different rates of expansion when heated. Exposed rock will 

 suffer great changes in temperature in twenty-four hours, 

 especially if it be located in a region of high altitude and 

 cloudless weather. A block of marble one hundred feet 

 long will expand one-half inch with a change of 75° Fahren- 

 heit, and this is frequently of diurnal occurrence in an arid 

 climate. Because the minerals composing rock expand and 

 contract at different rates, they tend to tear apart, thus 

 producing crevices that may fill with water, and this water 

 acts still further to disintegrate the rock. 



11. Action of frost. — One reason that building stones 

 are more likely to disintegrate in a cold moist climate than 

 in a dry or warm one is that the small pores and cracks on 

 their surfaces fill with water, which, when it freezes, exerts 

 an enormous pressure. The expansive power of freezing 

 water amounts to about 150 tons to a square foot, which is 

 equivalent to a column of rock a third of a mile in height. 

 The rock surface becomes chipped off by repeated freezing 

 and even great masses of rock are detached by the freezing 

 of water in larger cracks, as may be seen beneath rock ledges 

 in the spring of the year. 



An interesting example of the effect on rock disintegration 

 of a cold moist climate as compared with a dry one is found 

 in the difficulty that has been experienced in preserving 

 the obelisk, now in Central Park, New York, which had pre- 

 viously stood for many hundreds of years in the Egyptian 

 desert without great damage. It has been found necessary 

 to cover the entire surface of the stone with paraffine in 

 order to preserve the hieroglyphics carved on its surface. 



12. Action of water. — Water has another effect on rock. 

 It is a solvent, weak but universal. It acts on all minerals, dis- 

 solving slight quantities of some, considerably more of others. 



