190 



EOCK DISINTEGRATION AND DECOMPOSITION 



ering, may exist as a hydrous silicate of alumina only (kaolin). 

 It is in instances of this kind that the microscope may render 

 efficient service, and much may be learned by means of such 

 mechanical analyses as can be made by sifting and washing. 

 Such separations made on this disintegrated rock showed it to 

 consist of particles as given in the following table, the 4.25% 

 silt being obtained by washing the 10.75% of material which 

 passed through fine bolting-cloth of 120 meshes to the lineal inch, 

 and which represents the impalpable mud remaining in sus- 

 pension while the 6.5% of fine sand sank quickly to the bottom 

 of the beaker in which the washing was made. The residual 

 sand yielded then : 



Silt 4.25% Largest grains 0.1 mm. in diameter 



Very fine sand .... 6.50 



Fine sand ...... 11.25 



Medium sand 3.80 



c, T ( 11.00 



^^^M 23.50 



Coarse sand 29.50 



Gravel 10.20 





' 0.18 '' '' 





' 0.25 " *' 





' 0.65 '* '' 





' 1.00 '' '* 





' 1.50 '' *' 





' 2.00 '' '' 





< kCiC\ it << 



Total 



. 100.00% 



The coarser of these particles, like the gravel and coarse sand, 

 are of a compound nature, aggregates of quartz and feldspar, 

 with small amounts of mica and other minerals. In the finer 

 material, on the other hand, each particle represents but a single 

 mineral, the process of disaggregation having quite freed it from 

 its associates, excepting of course, the microscopic inclusions 

 which could be liberated only by a complete disintegration of the 

 host itself. These particles, as seen under the microscope, are all 

 sharply angular, and in many cases surprisingly fresh, though 

 the analyses had suggested only a slight change in chemical com- 

 position. The mica shows the greatest amount of alteration, the 

 change consisting mainly in an oxidation of its ferruginous con- 

 stituent, whereby the folia becomes stained and reduced to yel- 

 lowish brown shreds. The feldspars are, in some cases, opaque 

 through kaolinization, but in others are still fresh and unchanged 

 even in the smallest particles. The finest silt, when treated with 

 a diluted acid to remove the iron stains, shows the remaining 

 granules of quartz, feldspar, and epidote beautifully fresh, and 

 with sharp, angular borders, the mica being, however, almost 

 completely decolorized. 



