SURFACE DEPOSITS. 83 



but the variation is principally due to the varying pro- 

 portions of the component materials. These materials are 

 always incoherent; that is, they are not cemented nor con- 

 solidated into rock, but may always be excavated with more 

 or less facility. All the materials of the drift being so 

 intermixed, it is somewhat difficult to give any definite 

 description of each, especially of the finer ones. Snch 

 descriptions, therefore, mnst be merely general. 



Tlie clay of the drift, which is always present in greater or 

 less proportion, is always impure; always finely dissemi- 

 nated throughout the whole deposit, but not unfrequently, 

 irregular masses of it are separated from the other materials. 

 Its color is usually yellowish from the per-oxyd of iron it 

 contains, and which when it is burned into bricks gives them 

 their red color. 



The clay is frequently sufficiently separated from the other 

 materials, to assume the peculiar fissured character which sepa- 

 rates the mass into small, irregular, angular divisions, in 

 which condition it is popularly called "joint clay." Again, 

 particularly within the limits of the coal-field, it has frequently 

 the same character as that into which the clayey and shaly 

 beds of the coal formations are seen to become weathered, 

 where they crop out in the valleys of the streams of that 

 region. 



Much of the so-called clay of the drift, and also of the soil, 

 is however, not wholly a silicate of alumina, as true clay is, 

 but it is often largely intermixed with more or less pure 

 silica in a finely divided condition. Besides this, it usually 

 contains varying proportions of the carbonates and silicates 

 of lime and magnesia, the carbonates being much the most 

 abundant. 



The proportion of lime in the drift of Iowa, is so great that 

 the water of all our wells and springs is too " hard " for wash- 

 ing purposes ; and the same substance is so prevalent in the 

 drift clays that they are always found to have sufficient flux 

 when used for the manufacture of brick. 



It is frequently the case, that the sand and gravel have 



