452 



rhoades. CONCRETE AGGREGATE 



[Ch. 24 



types. Hence wind-blown sands, by and large, are composed pre- 

 dominantly of quartz. The grains are well-rounded and concentrated 

 in a few size grades which reflect the average transporting capacity 

 of the prevailing winds of the area. Such sands are useful in con- 

 crete aggregate only for "blending" to augment other aggregates which 

 are deficient in the finer size grades. A typical wind-blown sand, oc- 

 curring near Pot Holes Dam site, Columbia Basin (Washington) is 

 shown in Fig. 5. 



The gradation and composition of a typical wind-blown sand from 

 near Kortes Dam (Wyoming) is shown in Table 2, sample A. A de- 



TABLE 2 

 Grain Size of Wind-Blown Sands 



A. Kortes Dam area, Wyoming. Mainly quartz and feldspar; some amphibole, 

 mica, chlorite, chert, limestone, and siltstone. 



B. Columbia Basin, Washington. Derived from lavas. Composition is plagio- 

 clase feldspar, with smaller amounts of basaltic glass, basalt, quartz, olivine, acidic 

 glass, rhyolite, and opal. 



C. Pasco Pumping Plant area, Washington, near sample B. Derived from marine 

 ^nd glacial sediments that mantle adjacent Pasco slope. Composition is mainly 



quartz, feldspar, and muscovite. 



posit of wind-blown sand occurring in the Columbia Basin (Washing- 

 ton) is noteworthy by reason of its mineralogic and petrographic com- 

 position (sample B). Its heterogeneous composition results from a 

 predominantly local derivation from the Columbia Basin plateau lavas 

 which comprise the prevailing bedrock. However, sample C, another 

 blowsand from the same general area (near the Pasco Pumping Plant, 

 Columbia Basin, Washington) exhibits quite a different composition, 



