TNDUBATION OK EXPOSUEE 241 



place on the surface of exposed blocks of siliceous sandstone in 

 Wisconsin. ''The St. Peters Sandstone is composed almost 

 wholly of a pure quartz sand, and in the outliers of it found on 

 the hilltops south of the town, the parts covered by the soil were 

 more or less friable, and the grains distinct; while the exposed 

 portions of the same blocks and slabs were greatly indurated, 

 the grains almost obliterated, and the rock possessed the con- 

 choidal fracture and other characteristics of a quartzite." In 

 this and other cases cited by Dr. "Wadsworth, the cementing mat- 

 ter is silica. 



The explanation given (in letter to the present writer) is to 

 the effect that all water, including that of rains, as well as ter- 

 restrial, dissolves silica, which is again deposited under suitable 

 conditions. Part of the silica apparently comes from the solu- 

 tion of the quartz, chalcedony, and opal, and a part from the 

 alteration and destruction of the silicates. Both solution and 

 deposition seem at times to take place on the immediate surface, 

 the interior waters in such cases playing no part. 



P. Choffat regards it as possible that silica set free through 

 feldspathie decomposition in granitic rocks may, on eTaporation, 

 be redeposited in an insoluble form in the interstices of the fresh 

 rock in the immediate vicinity, thus retarding if not wholly 

 preventing further decay in that direction.^ 



Professor W. 0. Crosby, in a personal memorandum to the 

 writer, calls attention to the fact that in the disintegrated 

 granites of the Pikes Peak, Colorado, area, the rock is almost 

 invariably exceptionally firm and impervious along the joints, 

 indicating a local induration due perhaps to infiltration of iron 

 oxides or silica. Where a joint face bounds a ledge of rock, it 

 often maintains its integrity, weathering out in relief like a 

 quartz vein, while the granite is in a condition of advanced 

 degeneration all around. A slight break in the face of a joint 

 plane, in such cases, may lead to extensive disintegration behind 

 it, until it finally falls away from the disintegrating mass, a slab 

 of relatively sound rock. 



Andesitic rocks in regions of limited rainfall have been noted 

 by Professor G. Vom Rath as having become covered on the 

 upper surface with a thin layer of brown iron oxide, which pro- 



^Sur quelq-ues eas cVerosion atmospherique dans les garnites du Minlio, 

 Commimica^oes da Birec^ao Dos Trabalhos Geologicos de Portugal, Tome 

 3, Ease. I, 1895-96, p. 17. 

 17 



