28 



PHYSICAL GEOLOGY 



Fig. 



- Sandstone "set on edge," showing the 

 scaling of the laminae bv frost. 



soaked in water for 

 twenty-four hours and 

 again weighed, are found 

 to have gained one eighth 

 in weight. This fact 

 shows not only that pores 

 are present, but also that 

 they have become filled 

 with water. If such a 

 rock, with its pores full 

 of water, is frozen re- 

 peatedly, its grains will 

 be forced apart until it 

 finally falls to pieces. 

 This test is used in labo- 



ratories to determine the desirability of building stone in temperate 

 regions. The complete disintegration of the rock is not attained by 

 one freezing and thawing, but if the process is repeated many times, 

 the rock may be completely re- 

 duced to sand, as the water 

 penetrates farther into the rock 

 each time it thaws. It is 

 readily seen that this wedge 

 work of ice is usually more im- 

 portant in moist, temperate 

 regions in early and late winter. 

 The obelisk which now stands 

 in Central Park, City of New 

 York, stood for many centuries 

 in Egypt without apparent 

 injury (although undoubtedly 

 weakened by the extremes of 

 daily temperature to which it 

 had been so long subjected). 

 But after one year's exposure 

 to the moist, changeable climate 

 of its new home, the hiero- 

 glyphics near its base became 

 almost illegible, and it was 

 found necessary to coat the 



Fig. 4. — Much-fraetured limestone with 

 strong bedding planes, illustrating the con- 

 ditions favorable for the wedge work of 

 frost and roots. The beds are bent into a 

 low anticline. 



