» LAG GRAVELS. 



whole it is coiitiiiually left in the rear of these, whieh 

 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, as given in the table of anal3^ses (Tab. I) . It is not 

 likely that these few samples adequately represent the 

 composition of similarly formed deposits in other locali- 

 ties. The largest rock fi-agment in the lot measured only 

 a little over eight miillimeters 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 pre- 

 sent in four of the samples. All the other, with one excep- 

 tion, had pebbles over two millimeters in diameter. The 

 different grades are rather indiscriminately mingled, in a 

 manner determined by the caprices of the wind. 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 ninety per cent of 

 the weight is distributed among five different gi-ades ; in 

 six, among four grades; and in one, among three. In an 

 average of all ten samples ninety per cent of the weight is 

 distributed among five grades. The highest maximum 

 in any grade is sixty-eight per cent and the lowest is 

 twenty-five. The average height of the highest maxima 

 is forty per cent. 



The lag gra\-els are the most heterogenous of all the 

 wind deposits. Thej^ are generally distinctly stratified. 

 The fine admixtures are sometimes present as an original 

 constituent of the eroded ground, but they may some- 

 times also be deposited with the gravels by the lighter 

 winds. Compared with water-bedded materials of the 



