r/ze Saint Peter Sandstone. 85 



In the first place, with the exception of the very small 

 percentage of kaolin the impurities distributed through the 

 Saint Peter sandstone can be and evidently should be con- 

 sidered as recent infiltrations. Therefore the stone was once 

 an almost absolutely pure quartz. In thesecond place, there 

 are foSvSil casts in the white sandstone; these must have 

 been produced by calcareous shells, although the shells them- 

 selves have been dissolved away so thoroughly as to leave 

 no stain nor trace of their substance. Still other fossils are 

 moulds of former internal casts as already described. The 

 moulds are now full of loose sand, which together with sol- 

 uble materials, probably, for the most part, calcium carbon- 

 ate, formerly composed the internal casts from which the 

 moulds are preserved. The amount of quartz sand as com- 

 pared with the now missing soluble materials, seems to be 

 less than half the content of the moulds. The question sug- 

 gests itself: Could the strata of the Saint Peter have been 

 originally one-half calcium carbonate? Was the formation 

 originally a calcareous sandy mixture, which later became 

 pure quartz by the chemical action of water? 



The Saint Peter strata constitute a sandstone in the' 

 highest degree porous and water flows through it readily, so 

 that the very best circumstances for infiltration and exfiltra- 

 tion are now afforded by it. It could have been for long 

 periods a perfect underground waterway. It is, in fact, sit- 

 uated next under a series of clay, marl, and limestone forma- 

 tions that are more impervious to water, while beneath it 

 are sandstones and somewhat porous dolomites through 

 which water could pass somewhat readily. The exposed, 

 eroded, and glaciated border of these formations is at the 

 same time their more elevated portion, so that water sinking 

 into any of them would be carried deeper as it coursed in the 

 direction of the stratification. The water in the Saint Peter 

 would be held back by the impervious strata above it, while 

 that in the strata beneath could break upwards through the 

 dolomites and sands into the Saint Peter, which latter thus 

 would be the main underground w^aterway. This may ex- 

 plain why the sandstone is now so pure, for much calcium 

 carbonate and other constituents could have been carried 

 from the formation by coursing waters. 



