WIND DEPOSITS 677 



the action of the wind, it has not been transported very far. The de- 

 posits from which it has been derived are never far away. Commonly 

 it is a bank of sand, part of which has been removed. The finer grades 

 have been blown away, exposing these larger fragments to the force of 

 the wind, which apparently moves them by undermining and rolling. 

 It is evident that the coarseness of this gravel renders it much less sub- 

 .ject to the action of the winds than tlie finer materials. Occasionally it 

 may be found partly or wholly covered by finer materials, but on the 

 whole it is continually left in the rear of these, which follow the winds 

 with greater promptness. Only ten samples have been examined. These 

 were collected at eight different localities in the central part of the 

 United States. It is not likely that these few samples adequately repre- 

 sent the composition of similarly formed deposits in other localities. 

 The largest rock fragment in the lot measured only a little over eight 

 millimeters in its longest diameter. It was part of a sample consisting 

 of flat chips of a hard shale. Pebbles over four millimeters in diameter 

 were present in four of the samples. All the other samples, with one 

 exception, had pebbles over two millimeters in diameter. The different 

 grades are rather indiscriminately mingled in a manner determined by 

 the caprices of the wind and by the variability of the materials on which 

 the winds have worked. Five of the samples have two maxima each. The 

 chief ingredients vary from fine gravel through coarse and medium sand 

 to fine sand. In three of the samples 90 per cent of the weight is dis- 

 tributed among five different grades; in six among four grades, and in 

 one among three. In an average of all ten samples 90 per cent of the 

 weight is distributed among five grades. The highest maximum in any 

 sample is 68 per cent and the lowest is 25. The average height of the 

 highest maxima is 40 per cent. 



List of Samples in Table 21 



191. Chips of shale, Edgemont, South Dakota. 



192. From a "blow-out," Hooppole, Illinois. 



193. From a "blow-out," Hooppole, Illinois. 



194. From north of Mineral, Illinois. 



195. From the rear of a dune, Michigan City, Indiana. 



196. From the bottom of a "blow-out," Alliance, Nebraska. 



197. From the rear of a dune, Michigan City, Indiana. 



198. From wind-blown ground, Ardmore, South Dakota. 



199. From the rear of a dune. New Boston, Illinois. 



200. From a "blow-out," Alliance, Nebraska. 



Coarse drifting sand. — Lag gravels graduate imperceptibly into coarse 

 blown sand, which in the field always lies in front of the gravels, follow- 



