G60 J. A. UDDEN COMPOSITION OF CLASTIC SEDIMENTS 



ous deposits^ although they are often by writers classified as glacial, for 

 the reason that they have structural features and a geographical distri- 

 bution closely associating them with the till. Near places where glaciers 

 have abraded the bedrock till no doubt exists, which consists of chiefly 

 coarse material. It seems likely that till produced by mountain glaciers 

 is coarser than till resulting from continental glaciation. All samples 

 analyzed for this paper belong to the latter class. They are notably 

 uniform in composition. Their clastic texture is evidently a result not 

 of sorting, but of crushing. It is to be regarded as representing a cer- 

 tain stage in this crushing process for any particular sample. It would 

 seem that the tills analyzed have been ground to nearly identical stages 

 of fineness. The maximum ingredient is the same in five of the eight 

 samples, consisting of fragments from one-sixteenth to one thirty-second 

 millimeter in diameter. In one sample it is two grades coarser than 

 this ; in one, one grade coarser ; and in one, two grades finer. The grades 

 containing quantities which are measurable by this method of analysis 

 number from 13 to 16 in different samples, and would no doubt have 

 been a few more if the samples had been larger, for we know till con- 

 tains large boulders ranging up to many feet in diameter; but it is safe 

 to say that 99 per cent of the tills in Illinois and Iowa will not go much 

 outside of 16 grades. If we average^ these samples, we find that the 

 quantities are more than 1 per cent in 13 grades, from very coarse gravel 

 to coarse clay, inclusive. Three more grades are represented by less than 

 1 per cent. The coarse admixtures exceed the fine admixtures consider- 

 ably. In these samples there is 9 per cent more of coarse admixtures 

 than of fine. This excess evidently represents a part of the till which is 

 specially resistant to trituration in the ice. 



List of Samples in TaMe 1 



1. Yellow boulder-clay from six miles south of La Salle, Illinois. 



2. Unweathered Kansan boulder-clay, Davenport, Iowa. 



3. Blue boulder-clay, two miles north of Council Bluffs, Iowa. 



4. Yellow boulder-clay from one-half mile east of Dimmick, Illinois, 



3 In the discussion of this and all the following tables all averages referred to have 

 been obtained by adding the percentages of all maxima, then the fine admixtures of 

 first, second, and third, etcetera, order ; likewise the coarse admixtures of the first, 

 second, and third, etcetera, order, each separately, and then dividing the sums by the 

 number of samples. In samples having more than one maximum, the highest one is 

 taken as the chief ingredient and the others are treated as admixtures. In two or 

 three samples, where two or three maxima are nearly of the same size, the selection of 

 one to appear as the main ingredient has been arbitrary. For the tills a better way of 

 averaging would perhaps be to average the middle grade, or the grade as near as pos- 

 sible equally distant from the extreme grades, and then average the other ingredients 

 in successive order on either side. 



