218 C. W. HAYES — SOLUTION OF SILICA. 



humic acids which were evolved in the process of its decay. This case, 

 although suggestive, is not strictly comparable with those above de- 

 scribed, since the solution may have taken place long after the burial of 

 both pebbles and plants and under conditions somewhat different from 

 those found at the earth's surface. 



SURFACE INDURATION OF SAINT PETERS SANDSTONE IN WISCONSIN. 



It is frequently observed that pure quartz sandstones have a decidedly 

 different texture in their exterior and interior portions, the latter being 

 soft and porous, while the former is hard and dense. A specific case is 

 that described by Wadsworth,* who has shown that the Saint Peters 

 sandstone in the vicinity of Mazominie, Wisconsin, where covered, is fri- 

 able and composed of distinct grains, while in the exposed portions the 

 sand grains are almost entirely obliterated, the rock possessing the con- 

 choidal fracture and other characteristics of quartzite. 



SURFACE GLAZING OF SANDSTONES IN PENNSYLVANIA AND TENNESSEE. 



Again, the Medina sandstone in Pennsylvania and its southern equiva- 

 lent, the Clinch in Tennessee, often shows a marked superficial indura- 

 tion, which in some cases converts the outer portions into a vitreous 

 quartzite, and in others produces a thin surface glaze. 



From the conditions under which this induration takes place it appears 

 to be due to the solution of quartz in the interior . portions of the rock 

 and its transfer by capillary action to the outer portions, where the sol- 

 vent is evaporated and the silica redeposited in the interstices of the 

 sandstone. 



Chemical Activity of the azo-humic Acids. 



The wide separation of the localities at which these several cases of 

 undoubted solution of silica occur precludes the idea that they are due 

 to some peculiar and exceptional conditions, such as the presence of 

 thermal alkaline waters. It seems more probable that they must be 

 attributed to widespread agencies working in these cases under more than 

 ordinarily favorable conditions, so that the effect is exceptionally strik- 

 ing. All the cases described occur in a heavily forested region, where 

 there is an abundant layer of humus. In one case the pebbles receive 

 the drip from a cliff above which is a long wooded slope ; in another, the 

 stream which at times covers the conglomerate heads in rather extensive 

 swamps on the plateau and its waters are generally more or less deeply 



*M. E. Wadsworth : Some instances of atmospheric action on sandstone. Proc. Bost. Soc. Nat. 

 Hist., vol. xxii, 1883, p. 201. 



