126 



CARBONIFEROUS LIMESTONES 



below the rapids, boulders are much less abundant than they are immediately on 

 them. 



About a quarter of a mile above the Rapids, sandstones again appear in place 

 beneath the drift. Five miles higher up, and two to three miles below the Second 

 Correction Line, the shaly beds extend to the height of sixty-five feet above the 

 water-level, where they are overlaid by from twenty to thirty feet of heavy beds 

 of white and light-gray gypsum rocks, lying in horizontal, conformable beds, which, 

 at a little distance, might be taken for ledges of white sandstone. For thickness 

 and extent, this is by far the most important bed of plaster-stone known west of 

 the Appalachian Chain, if not in the United States. It is seen at intervals for 

 three miles, exposed on both sides of the Des Moines, in mural faces of from 

 eighteen to twenty-five feet, always overlying pink shales, from beneath which 

 copious springs of excellent water issue. It has been traced in the ravines, back 

 from the river, for nearly three-quarters of a mile, where it is finally lost under the 

 deep alluvion of the vast plains that stretch away to the west. There is every 

 reason to believe that it occupies an area of from two to three miles square, retain- 

 ing an average thickness of twenty feet ; perhaps double that thickness at certain 

 points. This plaster-stone of the Des Moines does not appear to have been depo- 

 sited in nests or conical heaps, as in the shales of the Onondaga Salt Group of 

 New York, but rather in continuous horizontal beds, conformable to the underlying 

 shale. The immense quantity of gypsum of this part of the Upper Des Moines, 

 can hardly be accounted for on the principle of a double chemical decomposition, 

 between sulphate of iron and carbonate of lime, formerly existing where the plaster 

 now is, since there does not appear to be an equivalent bed of iron in the vicinity, 

 nor yet beds of limestone, except thin bands of black, bituminous, calcareous rock, 

 by no means extensive, that are in immediate connexion with the plaster-beds. It 

 seems rather to have been an original deposit at the bottom of the ocean ; the 

 sulphate of lime having probably been derived, during the formation of the rocks, 

 from submarine sources. 



In mass, the plaster-stone* is white, with light shades of a yellowish gray colour, 

 running in horizontal streaks ; in powder, it is as white as flour. The texture is 

 fibrous; the horizontal face presents a stratified appearance. When ground or 

 burnt it sets well, becoming hard in the course of a few minutes. 



Everywhere in the region of the plaster-stone, the banks of the Des Moines are 

 clothed with an extraordinarily thick vegetation ; indeed, the undergrowth and vines 



* Its composition is — 



Sulphate of lime, . . . . . 70 '8 



Lime combined with silica (?), as silicate of lime, . 2-2 



Carbonate of lime, . . . . 2- 



Phosphate of lime, . . . . . 1-1 



Insoluble matter, . . . . . 2- 



Magnesia, ....... 0-7 



Water, ....... 20- 



Chloride and sulphate of alkali, .... -3 



99G 



