THE ORIGIN OF GUMBOTIL 103 



Percentage 



Limestone 40.0 



Granite . . .11.0 



Basalt and greenstone 27.0 



Felsite 3.0 



Sandstone i.o 



Slate 0.4 



Schist .0.4 



It is clearly seen that the content of siliceous pebbles of the un- 

 leached and oxidized Kansan till is considerably less than that 

 of the leached and oxidized Kansan till, the average siliceous 

 content of the former being less than 20 per cent. 



It is of interest to note that the kinds of pebbles found in the 

 different zones of material are much alike. Of course limestone is 

 in the imleached zone only. 



Similar results were obtained when the Nebraskan and Illinoian 

 gumbotils and their respective underlying tills were studied. For 

 instance, the average pebble content obtained from several analyses 

 of Nebraskan gumbotil is as follows : 



Percentage 



Quartz 36.75 



Chert, flint, etc. . 21.25 



Quartzite 20.25 



Granite 8.25 



Basalt and greenstone ii.o 



Feldspar 1.25 



Felsite . . . '. o . 50 



The content of siliceous pebbles is here more than 78 per cent; 

 none of the Nebraskan gumbotil examined gave less than 72 per 

 cent, and the highest gave 88 per cent. The studies of leached 

 and oxidized Nebraskan till gave about 38 per cent of siliceous 

 pebbles, and the unleached and oxidized Nebraskan till gave about 

 15 per cent of siliceous pebbles. 



Analyses of the pebbles of Illinoian gumbotil gave an average 

 result as follows : 



Percentage 



Quartz .43 



Chert, flint, etc 53 



Quartzite 



Granite 



Basalt and greenstone ........ 2 



Sandstone 2 



