242 THE PHYSICAL MANIFESTATIONS OF WEATHERINa 



tected them from further disintegration. Such crumbled away 

 only from the under surfaces, where they absorbed moisture from 

 the ground, and gave rise thus to peculiar tent-like and mush- 

 room-shaped forms. 



The present writer has noted in the Madison valley, north 

 of the Yellowstone Park, rounded masses of a vesicular rhyolite 

 which have, through the same causes, been reduced to the con- 

 dition of mere shells with openings on the under side and that 

 facing the direction of the prevailing winds. In these cases, 

 however, the wind seemed to have aided in their formation, not 

 merely through transporting the disintegrated material, but by 

 catching up and whirling about the loosened granules within 

 the gradually enlarging cavity, where, by force of impact, as 

 already described, they become themselves agents of abrasion. 

 Some of the cavities observed were of sufficient size to afford 

 shelter for a human being and had served as temporary dens for 

 wild animals. 



Roth mentions^ an induration evidently somewhat similar to 

 that described by Vom Rath above, as having taken place, on 

 the surface of a reddish yellow sandstone in Fezzan, North 

 Africa. The crust thus formed was so dense and hard as to 

 break with a shell-like fracture resembling basalt. A similar 

 incrustation on sandstone from the Lybian desert was found by 

 Zittel to consist of: manganese oxide, 30.57%; iron oxide, 

 36.86%; alumina, 8.91%; silica, 8.44%; barium oxide, 4.89%; 

 sulphuric acid, 4.06% ; phosphoric acid, 0.25% ; and water, 5.90%. 



The Potsdam quartzites of Minnesota have had, in many in- 

 stances, an almost glass-like polish imparted to their exposed 

 surfaces through no other apparent agency than that of wind- 

 blown sand. Unlike a polish produced by artificial methods, 

 this wind polish extends to the bottoms of every little groove 

 and cavity, or over every protruding knob alike. In softer 

 rocks, or rocks of less homogeneous structure, the same agencies 

 carve out the softer portions, leaving the more resisting pro- 

 truding, as already described on p. 163. This polish is so per- 

 fect, even on rough surfaces, as to suggest a partial solution of 

 the granules, and a redeposition of the dissolved matter in the 

 form of a glaze, but the microscope proves to the contrary. 

 The gloss is due wholly to superficial smoothing and no new 



^ Allegemeine u. Ohemisehe Geologie, 2d eel., Vol. Ill, p. 215. 



