GEOLOGY OF SOUTHWESTERN IOWA. 361 



hardly be otherwise than that all three of the coal-measure 

 formations have thickened in some degree at least from the 

 middle of their onter border in Iowa, in which case the boring 

 at Nebraska City evidently came much short of reaching the 

 base of the Lower coal-measures, even if the drill entered that 

 formation at all, where alone we may hope to find the 

 thickest beds of coal. Even if no thickening of the formations 

 occurs in that direction, the Nebraska City boring was carried 

 hardly deep enough to reach the base of the Lower coal- 

 measures, (in which relative position the heavy beds of coal 

 exist in the valley of the Des Moines), because the aggregate 

 thickness of the Middle and Lower coal-measures as actually 

 measured in Iowa, is greater than the depth of the boring. 



Material Resources. The soil of Fremont county is an 

 inexhaustible source of wealth, for it is almost wholly com- 

 posed of that fine lacustrine material known as the Blun 

 Deposit, which is perfectly homogenous from top to bottom, 

 and when ^thrown out upon the surface from a hundred feet 

 below it, its fertility is almost as great as that already upon 

 the surface. Its mineral resources consist mainly of its stone 

 so far as their existence has yet been demonstrated. Large 

 quantities of stone may be obtained in the northwestern 

 quarter of the county, the exposures being found only at the 

 base of the blufTs that border the flood-plain of the Missouri 

 river. The smaller exposures at Plum Hollow and below, as 

 far as Hamburg, furnish considerable quantities of stone for 

 ordinary use, but those extensive exposures at Wilson's, and 

 at other points to the northward, contain much that is fit for 

 dressing into caps and sills for buildings, and also excellent 

 material for lime. 



Brick clay, properly so called, is quite scarce in Fremont 



county, and yet, very good bricks are made from material 



obtained from the base of the Bluff Deposit, where it joins 



the drift and has received some admixture of its clay. Even 



although the clay contained may amount to only a small 



fraction of the bulk, the lime it always contains seems to act 



as a sufficient flax in burning to cement the minute silicious 

 46 



