THE ORIGIN OF GUMBOTIL 105 



Similar studies made of pebbles taken from Kansan gumbotil 

 and from underlying Kansan drift gave results as follows: largest 

 pebble in gumbotil 3.2 cm., smallest pebble 3 mm., and average 

 pebble 7 mm. In the underlying drift there are many pebbles 

 10-12 cm. in diameter, a few more than 3 cm. in diameter; the 

 smallest pebble seen was 7 mm., and the average of one hundred 

 pebbles collected was about 1.8 cm. The shapes of the pebbles 

 were similar to corresponding horizons in the Nebraskan materials. 



CHEMICAL STUDIES OF GUMBOTIL AND RELATED MATERIALS 



In addition to a study of the field relations of Nebraskan 

 gumbotil, Kansan gumbotil, and Illinoian gumbotil, the tills which 

 underlie these gumbotils, and laboratory studies of the physical 

 properties of these materials, there were made detailed chemi- 

 cal analyses of gumbotils and related materials. The speci- 

 mens were taken from exposures which had been studied carefully 

 in the field, and the materials selected were thought to represent 

 satisfactorily the compositions of the zones from which they were 

 taken. The analyses were made from i-gm. samples of fine 

 material which had been separated carefully from pebbles and 

 concretions. Only the material which could be sifted through a 

 "twenty-mesh" copper-gauze sieve was pulverized and subjected 

 to chemical analysis. Accurate determinations were Hmited to 

 the oxides of aluminum, silicon, iron, calcium, and magnesium, 

 since deductions as to the nature of the chemical processes involved 

 in the transformation of the drift can be made only upon the pro- 

 portions of these less mobile constituents now present. The 

 analyses were made in strict accord with the preferred methods and 

 the recommendations prescribed by Hildebrand.^ 



Before referring in detail to the kinds of materials which were 

 analyzed, the localities from which they were taken, and the 

 results of the analyses, it seems well to discuss somewhat fully 

 some of the geo-physico-chemical factors which need to be under- 

 stood in order to interpret correctly whether or not a material such 

 as gumbotil is the product of weathering of till. Chemical evi- 

 dence will be presented to support the field evidence that the 



' Hildebrand, "The Analysis of Silicate and Carbonate Rocks," U.S. Geol. Surv., 

 Bulletin 422. 



