678 J. A. UDDEX COMPOSITION OF CLASTIC SEDIMENTS 



ing the direction of the prevailing winds. Farther in this direction the 

 coarse sand becomes in turn finer and finer, until the main deposit is 

 reached, where it always consists of grains of a more uniform size. In 

 fact, the main bnlk of all sand drifts large enough to be called dunes 

 have been found to contain only subordinate proportion of sand grains 

 measuring more than one-fourth or less than one-eighth of a millimeter 

 in diameter. 



Sand coarser than typical dune sand is present as a maximum in- 

 gredient only in superficial layers of no very great thickness, which lie 

 on the rear slopes of dunes. It forms an intermediate series between 

 typical dune sand and lag gravels, and it is capable of being rolled rather 

 than lifted by the winds. This is indicated by the circumstance that it 

 is often the main ingredient on the crests of wind ripples, being heavy 

 enough to remain resting in this exposed position, while the finer dune 

 sand is lifted to the upward slope of the next ripple. It differs from the 

 lag gravels in being light enough to be rolled up a gentle slope and to be 

 moved without any undermining 'taking place. Eleven samples of such 

 rolling blown sand, as it may be called, have been collected and exam- 

 ined. Its composition is much more uniform and regular than that of 

 lag gravels. The proportions of the different grades arrange themselves 

 in all the samples in two decreasing series on either side of a maximum 

 which in three cases consists of coarse sand and in eight cases of medium 

 sand. In one sample the maximum grade constitutes 85 per cent of the 

 whole sample. The smallest maximum is 34 per cent. All the maxima 

 average 50 per cent in the 11 samples. Xinety per cent of each sample 

 is distributed among only three grades in nine of the samples and among 

 four grades in the other two. 



List of Samples in Talle 22 



201. Rear slope of a dune, Alliance, Nebraska. 



202. Rear slope of a dune, Tampico, Illinois. 



203. Rear slope of a dune, New Boston, Illinois. 



204. Rear slope of a dune, Alliance, Nebraska. 



205. Rear slope of a dune, Alliance, Nebraska. 



206. Rear slope of a dune, Hooppole, Illinois. 



207. Rear slope of a dune, Hooppole, Illinois. 



208. Rear slope of a dune, Hooppole, Illinois. 



209. Sand drift, Griggs County, North Dakota. 



210. Sand drift, Griggs County, North Dakota. 



Dune sand.— The sand which constitutes the main body of dunes has 

 been found remarkably uniform in its mechanical composition. Thirty- 

 eight samples have been analyzed, coming from 11 different localities. 



