LOWER SILTJEIAISr SYSTEM. 171 



contains is fresh and pure, does not adhere with sufficient 

 firmness to make a solid wall. It is however the only kind of 

 stone to be had in all the region where it occurs, and although 

 not the best, it is still very valuable. 



Geological Age. No traces of fossil remains of any kind 

 have yet been found in the Sioux Quartzite, or in its 

 associated pipe-stone. Therefore we have no other than 

 stratigraphical and lithological evidences of its geological 

 age. The following is a summary of the reasons which induce 

 its reference to Azoic age : 



1. Its complete metamorphic character; while the other 

 stratified rocks, at least in Iowa, down to and including the 

 Potsdam sandstone, are free from true metamorphism. 



2. Its disturbed condition; while all the other formations 

 just named are at most but slightly inclined, and are all 

 nearly or quite conformable to each other up to the close of 

 the Carboniferous age at least. No formation has been found 

 resting conformably upon the quartzite. In other words we 

 know of no such disturbance of strata as the quartzite has 

 suffered, having taken place in this part of North America, 

 between the commencement and the close of the Palaeozoic 

 time. 



3. It is known to be older at least than the Lower magne- 

 sian limestone, because that formation, near New "Dim, rests 

 without perceptible disturbance, and of course unconformably 

 upon the disturbed quartzite ; and because the Lower magne- 

 sian limestone has never been found to be unconformable with 

 the Potsdam sandstone, it is inferred that the Sioux Quartzite 

 is older than the latter also. 



2. LOWER SILURIAN SYSTEM. 



PEIMOEDIAL GEOTTP. 

 THE POTSDAM SANDSTONE. 



Area and General Characters. Although the Potsdam 

 sandstone has a geographical range extending throughout 



