Wadsworth.] 202 [February 7, 



That ordinary atmospheric agencies produce a greater effect 

 upon rocks of a siliceous character than is generally believed, I 

 have long deemed probable. This conclusion has been derived 

 from field observations especially upon sandstone and quartzites. 



As examples of the power of the ordinary atmospheric agencies 

 over rocks I would cite some cases observed by me in 1871, '72 

 and '73 near Mazomanie, Wisconsin, in the friable St. Peter's and 

 Potsdam sandstone. The St. Peter's sandstone is composed 

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

 on the hill tops south of the town, the parts covered by the soil 

 were more or less friable and the grains distinct ; while the ex- 

 posed portions of the same blocks and slabs were greatly indu- 

 rated, the grains almost obliterated, and the rock possessed the 

 conchoidal fracture and other characteristics of a quartzite. 



The Potsdam sandstone east of the town likewise possessed 

 similar characteristics. In this, concretions of an indurated char- 

 acter were found forming on the surface, in size from three-fourths 

 of an inch to an inch in diameter ; while in the sandstone a few 

 inches only their rudiments could be observed in minute 

 spherules, and a few inches farther in the sandstone was 

 incoherent, easily crumbling under the touch and showing no 

 trace of concretionary structure. At another locality the sand- 

 stone showed in cavities formed by weathering a distinct lining 

 of quartz crystals, while a few inches beneath the surface the rock 

 had the common friable character. The formation of these crys- 

 tals, as well as the surrounding surface induration of the rock, is 

 to be attributed to atmospheric agencies; as hot waters would 

 have consolidated the sands beneath, and not as is seen so exten- 

 sively in that vicinity only the surface rock left by denudation. 

 In the autumn of 1872 a block of clear white Potsdam sandstone 

 was found on the side of a hill, the protected side of which was 

 friable, while the other sides, especially the one most exposed to 

 the prevailing storms, was nearly a quartzite. This block was 

 only about two feet square, and as a test of the correctness of the 

 above conclusion the indurated surface was broken off, and a 

 comparatively friable surface exposed. This locality was visited 

 the following spring when it was found that this fresh surface 

 was much, indurated and approached towards a quartzite. 



This change had taken place in the few months that had 

 elapsed since the fractured and friable surface had been exposed 



