664 J. A. UDDEN COMPOSITION OF CLASTIC SEDIMENTS 



siderable variation in their composition. The maximum ingredient is 

 coarse silt in the coarsest sample and very fine silt in the finest sample, 

 showing a range through four grades. 



List of Samples in Table 6 



46. Glacial silt from a single layer, Clinton, Iowa. 



47. Glacial silt from a single coarse layer, Clinton, Iowa. 



48. Glacial silt, coarse and fine layers mixed, Davenport, Iowa. 



49. Glacial silt from a single coarse layer, Davenport, Iowa. 



50. Glacial silt from a single layer, Clinton, Iowa. 



51. Glacial silt from a single layer of fine texture, Clinton, Iowa. 



52. Glacial silt from a single layer, Clinton, Iowa. 



53. Glacial silt from a single layer, Davenport, Iowa. 



54. Glacial silt from a fine layer, Davenport, Iowa. 



55. Glacial silt from a single layer of fine texture, Clinton, Iowa. 



Sediments in present streams. — Some samples have been taken of sedi- 

 ments made in streams, where the conditions of sedimentation are some- 

 what better kno^ai than in the case of terrace deposits and deposits in 

 glacial waters. The composition of these samples is best presented in 

 four groups : 



1. Gravel and sand in small streams. 



2. Silt in small streams. 



3. Gravel and sand in large streams. 



4. Silt in large streams. 



These deposits are formed under a great diversity of conditions, and the 

 few analyses made do not adequately represent the great number of 

 variations of deposits. Especially is this true of the coarser deposits 

 such as gravel and shingle. It is not believed that there is a sufficient 

 number of analyses to show the true characteristic distribution of the 

 admixtures in the average of all these samples. Nevertheless, these 

 samples show a similarity to the terrace and glacial deposits of the same 

 class. 



Sediments in small streams.— In the coarser deposits in small streams, 

 neglecting the grade next below the maximum, there is an excess of 

 coarse admixtures. It is probable that the precise maximum, as we may 

 call that diameter which would divide the materials in these samples in 

 two equal parts, lies in this case nearer the lower limit of the maximum 

 grade than, the higher limit. 



In four samples of alluvial silt, collected in small streams, it appears 

 that these silts have not had time to be separated from coarser materials 

 as thoroughly as is common in most other silts. The coarser admixtures 

 exceed the finer by 13.5 per cent. 



