PRINCU'LES OF ROCK WEATHERING 853 



tain some 0.37 j)cr cent, by weight; chalk, 20 per cent.; 

 ordinary compact limestone, 0.5 per cent, to 5 per cent. ; marble, 

 about 0.30 per cent., and sandstones varying amounts up to 10 

 or I 2 per cent., while clay may contain nearly one-fourth its weight. 

 At and near the surface the amount, particularly after rains, may 

 be very considerably increased. This water is largely interstitial 

 — the quany ivater, as it is sometimes called. In addition to this 

 the quartz, particularly of granitic rocks, almost universally con- 

 tains innumerable minute cavities partially filled with water, and 

 which are in extreme cases so abundant as to make up, according 

 to Sorby, at least 5 per cent, of the whole volume of the mineral. 

 That the passage of this included moisture from the liquid 

 to the solid state mav be attended with results disastrous to the 

 stone is self-evident, though the rate of disintegration may be so 

 slow under favorable circumstances as to be scarce noticeable. 

 Freezing of the absorbed water is one of the most fruitful sources 

 of disintegration in stones confined in the walls of a building, 

 and even in the quarry bed it is b}" no means uncommon to have 

 the material so injured as to render it worthless. However 

 slight may be the effects of a single freezing upon a rock, con- 

 stant repetition of the process cannot fail to open up new rifts, 

 and still further widen those already in existence, allowing 

 further penetration of water, which freezes in its turn, and 

 exerts a chemical action as/well. So year in and year out, 

 through winter's cold and summer's heat, the work goes on until 

 the massive rock becomes loose sand to be caught up by winds 

 or K;mporarv rivulets and spread broadcast over the land. In 

 some instances, it may be, the rock is of sufficiently uniform 

 texture to be affected in all its mass alike. More commonly, 

 however, it is traversed by numerous veins, joints or other lines 

 of weakness along which the rifting power is first made manifest. 

 Naturally disintegration of this kind is confined to frigid and 

 temperate latitudes. As bearing upon the extreme rapidity with 

 which such disintegration may take place, I quote the following 

 fiom a letter of Dr. L. Stejneger, of the Smithsonian Institution, 

 who passed several months among the islands of Bering Sea: 



