102 GEORGE F. KAY AND J. NEWTON PEARCE 



The average pebble content of Kansan gumbotil gained from 

 eight analyses of pebbles made in widely separated areas in Iowa 

 is as follows: 



Percentage 



Quartz 48.5 



Chert, flint, etc 31.8 



Quartzite . . '. 6.8 



Granite 7.8 



Basalt and greenstone 2.9 



Feldspar i.o 



Sandstone 0.5 



It will be seen that more than 87 per cent of the pebbles are of 

 siliceous material; the highest percentage of siliceous pebbles 

 shown by any of the exposures of Kansan gumbotil subjected to 

 study was 98 per cent, the lowest 75 per cent. 



The average pebble content of the leached and oxidized Kansan 

 till beneath the Kansan gumbotil is as follows: 



Percentage 



Quartz .16.8 



Chert, flint, etc . . 16 . 5 



Quartzite 8.0 



Granite 20.3 



Basalt and greenstone 24.5 



Feldspar i.o 



Felsite 7.0 



Sandstone 1.0 



Shale 0.6 



Quartz porphyry 0.5 



Schist 2.3 



Gneiss 0.3 



The average content of siliceous pebbles is here only about 42 per 

 cent, compared with 87 per cent in the Kansan gumbotil; the 

 highest siUceous content was about 55 per cent, the lowest about 

 25 per cent. 



A study of the pebble content of the unleached and oxidized 

 Kansan till beneath the leached and oxidized Kansan till gave an 

 average result as follows, seven analyses being used: 



Percentage 



Quartz 6.4 



Chert, flint, etc 8.3 



Quartzite 3-° 



