200 The American Geologist. tpril, i&96 
northeast of Postville. Its upper surface, as determined by 
surveys made in connection with the topographic work on 
Allamakee county, has an elevation of 877 feet above sea 
level. Other exposures in the same neighborhood show that 
the strata here dip to the southwest at the rate of twelve to 
fifteen feel to the mile. Assuming the greatest observed dip 
to be correct and constant, the elevation of the Saint Peter al 
Postville should be 757 feet. Exposures of the upper surface 
of the sandstone at Volney, Smithfield and Werhan's mill, 
have a dip which, if projected to Postville, would make the 
elevation of the sandstone a few feet greater than the figures 
given above. The elevation of the mouth of the well is 1,11)1 
feet above sea level, and hence the depth from the surface at 
which the sandstone might reasonably be expected, judging 
from its position and dip in the nearest natural exposures, can 
not much exceed V)i feet. The point at which the sand was 
actually reached had a depth beneath the surface of 435 feet. 
"I'll • depth accords well with all field observations and is in- 
consistent with none. 
On the other hand, however, if the arenaceous bed at -fib") 
feet from the surface is Saint Peter sandstone, then we are 
confronted with the anomaly of sixty feet, or more, of 
material in all respects similar to the argillaceous and cal- 
careous shales of the Trenton, apparently interstratilied with 
the sand of the Saint Peter, and occupying the greater part 
of the space that properly belongs to this formation. No- 
where in the natural exposures, so far as observed, are there 
any indications of calcarerous shales interbedded with Saint 
Peter sandstone. Field records do, however, show that the 
Saint Peter is especially liable to become cavernous. The 
materials composing the deposit are so incoherent that 
mechanical erosion acts with great effectiveness in dislodging 
and transporting the rounded, polished, uncemented, quartz 
grains. Since the region was elevated above sea level, under- 
ground waters have cut channels in the sandstone, and some 
of these channels were formed near the base of the formation. 
Owing to the lo^ degree of cohesion among the constituent 
grains, gravity alone is sufficient to cause the walls and roof 
of any one of these channels to crumble and fall. The sand 
so falling is removed by the subterranean current. More takes 
its place from above to be in turn carried away; and thus in 
