Academy of Sciencj]. 11 



into masses of considerable size, and others still most perfectly rhomboidal crystals 

 upwards of a pound in weiglit. Peculiar incrustations are also noticed on the face 

 of the partially decomposed rock, in some cases in the form (^f a white flaky 

 powder, having the taste of common alum; in others in the form of efflores- 

 cent crystals giving the marked flavor of Epsom salts. As already intimated the 

 superstitious denizens of this region at once leaped to the conclusion that a young 

 volcano had burst upon them in all its fury, and fortified themselves with due 

 courage to meet the emergency. Attracted by their somewhat exaggerated ac- 

 counts, Mr. A. W. Barber, the editor of the Yankton Press, visited the locality and 

 made a careful and intelligent examination. It is to his courtesy that I am indebted 

 for an interesting box of specimens of the minerals here undergoing such vigorous 

 transformations, and for a full account of the nature of the bluff". A chemical 

 examination of these specimens reveals at once the nature of the interesting trans- 

 formations here in progress, which, while known to be actively in operation in 

 many localities in Europe, have as yet been observed at comparatively few points 

 within the limits of the United States. 



This so called "hot bluff," then, consists of two separate and mineralogically 

 distinct ingredients. The main portion of the bluff from the base up to the height 

 of a hundred feet consists of a bluish gray rock of uniform texture, somewhat 

 schistose, soft and easily yielding to the knife, and to the eroding action of the 

 river. So evenly balanced are its constituents that it seems a matter of question 

 whether it should be properly called an argillaceous limestone, or a calcareous 

 slate or argillite. For while upon analysis it is found to contain but about twenty 

 per cent, of the earthy carbonates of calcium and magnesium, it yet offers all the 

 characters of a magnesio argillaceous limestone, effervescing but slowly in cold 

 acids, but with quite violent action upon elevation of the temperature. 



Next above this the upper and remaining fifty feet of the blutf is completely 

 interwoven with seams of varying thickness of a soift, friable, blue-black shale, in 

 which exist in great quantity microscopic crystals of the bi-sulphide of iron (Fe 

 S2), cmnmonly known as iron pyrites. Nuggets of the same mineral are also found 

 abundantly distributed over the face of the blutf. Now it is the products of the 

 decomposition of this iron pyrites which form the active agents of all the violent 

 chemical reactions here observed. We know that when exposed to the influence of 

 atmospheric oxygen and moisture this mineral is rapidly decomposed, absorbing 

 oxygen and becoming converted into Ferrous Sulphate, (Fe SO4), commonly known 

 as green vitrol, and free sulphuric acid. Represented by formulaB, this change 

 would be: Fe S3 -f H3O + 70 -= Fe SO4 -f- H2SO4. This Ferrous Sulphate 

 again on more extended exposure is still further oxidized and becomes Ferric 

 Sulphate or Sesqui-Sulphate of Iron (Fe2 3S O4). This in its turn is a very 

 unstable compound, and when brought in contact with other bases readily yields 

 up its sulphuric acid and becomes converted into the hydrated oxide of iron. 

 When exposed in ' considerable mass, this decomposition of iron pyrites of course 

 proceeds but slowly, and specimens are frequently found illustrating the cliange in 

 all its gradations. But when, as in the soft shale of this Nebraska bluff', it is found 

 in the form of fine crystals appearing as glistening specks to the naked eye, its 

 decomposition is very rapicl and an abundance of free sulphuric acid is produced. 

 Hence it will hardly be a matter of surprise when I state that the soft shale from 

 the upper layers of this blutf has become so charged with free sulphuric acid as to 

 be strongly sour to the taste and acid in reaction. In fact, on allowing a specimen 

 of this shale to remain undisturbed in my laboratoiy for several months it was 



