WEATHEEINa 01^ DIABASE 



203 



Chatham, Virginia, analyzed and described by Dr. Thomas L. 

 Watson.^ The roek in its fresh state is dark-gray, homogeneous, 

 of medium texture, showing to the naked eye feldspars and 

 augites, but under the microscope an abundant sprinkling of 

 olivine and magnetite, some biotite and secondary serpentine and 

 chlorite. The feldspar was shown by analyses to be labradorite. 

 The rock in weathering breaks down into the usual boulder 

 masses, the transition from the bright orange residual clay to the 

 hard fresh roek being quite sharp, so that it is possible to ob- 

 tain hand specimens showing within the space of a few inches 

 all stages of the process. 



The material analyzed, as given on p. 204, was taken from the 

 outer portion of a small boulder such as were scattered through- 

 out the mass of residual incoherent clay, and concerning the 

 origin and derivation of which there could be no question. This, 

 although of a nature to be called, on casual inspection, an ochre, 

 showed on close examination a spongy mass of iron sesquioxide 

 through which was distributed a perfect network of white kao- 

 linized masses of the original feldspar. To the unaided eye the 

 mass seemed thoroughly decomposed without any trace of the 

 original silicate minerals preserved, but after removing the iron 

 oxide by continued digestions with very dilute hydrochloric acid, 

 and the residue subsequently examined under the microscope, 

 considerable traces of both undecomposed feldspar and augite 

 were distinctly recognizable, with surprisingly large quantities 

 of magnetite. Apparently, the magnetite was in as fresh con- 

 dition and in as large quantities as in the fresh and unaltered 

 rock. A mechanical analysis yielded the results given below : 



Diameter in mm. 



Conventional Names 



Feb Cent. 



(1) 2-.1 



Pine gravel. 



0.00 



(2) 1-.5 



Coarse sand. 



3.21 



(3) .5-.25 



Meditim. sand. 



13.10 



(4) .25-.1 



Fine sand. 



15.39 



(5) .1~.05 



Very f ne sand. 



23.49 



(6) .05-.01 



Silt. 



23.98 



(7) .01--.005 



Fine silt. 



4.16 



(8) .005-.0001 



Clay. 



14.20 





Total 



97.53 



^American Geologist, ToL XTI, 1898, p. 85. 



