I04 GEORGE F. KAY AND J. NEWTON PEARCE 



Analyses of pebbles of leached and oxidized Illinoian till beneath 

 Illinoian gumbotil gave results as follows : 



Percentage 



Quartz 28 



Chert, flint, etc 38 



Quartzite . 



Granite 12 



Basalt and greenstone 14 



Feldspar . 2 



Felsite 5 



Sandstone i 



These analyses show also that the percentage of siliceous pebbles 

 in the gumbotil is much higher than that in the oxidized and 

 leached till which underlies it. 



The sizes of pebbles in gumbotil and underlying drift. — ^When 

 pebbles from the gumbotils and underlying tills were being taken 

 in the field the only purpose in mind was to ascertain the percentage 

 content of the different constituents. Later, when these studies 

 were being made in the laboratory, it became evident that the 

 pebbles might also be used in estimating the relative sizes of the 

 pebbles in the different horizons, and in determining the shapes of 

 the pebbles. 



One hundred pebbles collected from the Nebraskan gumbotil at 

 one locaHty had dimensions as follows: largest pebble 2 .4X1 .4X1 

 cm., smallest pebble 2 mm., and average pebble 8X5 mm. The 

 shapes of these pebbles were subangular to spheroidal. The 

 unleached and oxidized Nebraskan drift beneath the Nebraskan 

 gumbotil had pebbles with dimensions as follows: largest pebble 

 6.25X4.5X1.75 cm. , smallest pebble 3X4 mm. , and average 

 pebble 1.75x1.5x1cm. The pebbles were chiefly flat and 

 subangular; some were slightly rounded. 



The largest pebble in the Nebraskan gumbotil from another 

 locality was 10x7x5mm., the smallest 1.5x2 mm., and the 

 average 3 mm. ; the shapes were subangular to spheroidal. Here 

 the underlying Nebraskan till had pebbles, the largest pebble of 

 which was 5.5X3 .5X3 crn., the smallest pebble 3x2 mm., and 

 the average pebble i.5XiX.75cm. The shapes of the pebbles 

 were subangular to more or less flat. 



