SEDENTARY MATEEIALSi EESIBUAEY DEPOSITS 297 



Delaware and southwestern Pennsylvania are mainly decom- 

 posed, MgMy f eldspatliic, gneissic rocks, which as dug from the 

 pits still retain their gneissic structure, but are now plastic clays 

 full of angular quartz fragments, mica scales and feldspar par- 

 ticles in various stages of decomposition. The change that has 

 taken place consists in a kaolinization of the feldspars, whereby 

 the alkalies are largely removed, and a residue consisting essen- 

 tially of a hydrous silicate 

 of alumina left in their 

 place. The quartz gran- 

 ules are disaggregated, 

 and their surfaces some- 

 times slightly etched by 

 the action of the alkaline 

 carbonates ; the black 

 mica, where such existed, 

 decomposed, giving rise 

 to rust-colored spots. 

 The material is dug from 

 the pits and washed with 

 water to separate the im- 

 purities, the "kaolin" or 

 elay remaining in suspen- ^f 29.-SliowiBg, on the left, the mineral 

 ^ ^ kaolmite as seen under the microscope, 



and on the right, washed kaolin. 



sion, and being ultimately 

 saved by filtration through 

 canvas. This finest material, as seen under the microscope, still 

 contains particles of undecomposed feldspars and shreds of white 

 mica, together with other extremely irregularly outlined, some- 

 times almost amceba-shaped forms, as shown in Fig, 29. An 

 average of two mechanical analyses of this clay, made under Dr. 

 Whitney's direction, yielded the results given below: 





Moisture in Air- 

 Dry Material at 

 100'^ 0. 



MoiSTtrEE ON 



Ignition 



Silt 



.05-.01 MM. 



Fine Silt 

 .01-.006 MM. 



Clay 

 .005-..0001 MM. 



0.41 0/, 



11.41 % 



31.79 % 



7.310/, 



47.78% 



Chemical analyses of the same material, made in the laboratories 

 of the United States Geological Survey, yielded : 



