442 A TREATISE ON METAMORPHISM. 



of years the weathering forces in Egypt, where diurnal temperature changes 

 are great, began so rapidly to disintegrate under conditions of freezing and 

 thawing that it had to be coated with paraffin to exclude water and save it 

 from complete destruction;" but in this case it should be remembered that 

 the effect of change from water to ice is superimposed upon the effect of 

 past centuries of insolation. Perhaps its rapid disintegration was due to 

 minute openings between the grains, formed by insolation, which the water 

 could enter. It is rather probable that stone fresh from the Egyptian 

 quarry would not disintegrate so rapidly. 



When water is in large, deep clefts, such as joints, and freezing takes 

 place, the result is to widen the cracks. Furthermore, it is to be noted that 

 at times of alternate freezing and thawing the process is cumulative. After 

 the openings have been somewhat enlarged and the ice melts, the rock does 

 not fully return to its original position, and when freezing again occurs the 

 widening again takes place, and so on. Thus the process may begin with 

 the subcapillary openings and by repeated freezing and thawing widen them 

 to large openings. At each time of expansion the ice acts as a wedge, and 

 besides widening the cracks it may also extend them, and thus the process 

 of freezing and thawing enlarges cracks already formed and also extends 

 them. 



In addition to the size and extent of the openings and the amount of 

 contained water, further factors affecting disintegration by freezing and 

 thawino- are topography and latitude. In proportion as the slopes are 

 sharp and the elevation is great the effect is likely to be great. The dis- 

 rupting effect is especially strong on cliffs and mountain sides and is slight 

 where the topographic variation is slight. It has already been seen that 

 the depth to which the effect may be produced varies from nothing to a 

 number of meters, and thus it is confined, under the most favorable circum- 

 stances, to the mere outer skin of the earth. The great effectiveness of the 

 change from water to ice in disintegrating rocks on steep slopes is clue to 

 the fact that the material disrupted is quickly pulled away from the parent 

 rock by gravity or is transported elsewhere by moving water or ice and a 

 new surface is exposed to the action of freezing water. 



Illustrations of rapid disintegration due largely to freezing and thaw- 

 ing are afforded by many of the higher mountain slopes, if not by nearly 



a For discussion of relations of size of pores to destructive effect by freezing and thawing, see Buck- 

 ley, cit., pp. 20-25, 382-384. 



