BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 



Vol. 6, pp. 321-332, PL. 16 MARCH 28, 1895 



DISINTEGRATION OF THE GRANITIC ROCKS OF THE 

 DISTRICT OF COLUMBIA 



BY GEORGE P. MERRILL 



{Read before the Society December 29^ 189 J^) 



CONTENTS 



Pa^e 



Introduction 321 



Description of locality 322 



Analyses and their discussion 323 



Bulk analyses 323 



Analyses of material separated by solvents . 324 



Analyses of material mechanically separated 325 



Conditions affecting the results 327 



Analyses of material from other localities 328 



Time-limit of disintcirration 328 



Causes of disintegration 331 



Introduction. 



The belt of ancient crystalline rocks bordering along the east side of 

 the Appalachian system south of the glacial limit affords abundant oppor- 

 tunity for the study of rock disintegration and decay as manifested 

 through the s(;me\vhat complex processes commonly grouped under the 

 term " weathering." The small area comprised within the northwestern 

 portion of the District of Columbia is particularly favorable to the observa- 

 tion and study of the chemical and physical processes involved. This is 

 due, first, to the fact that in numerous instances one is enabled to study 

 all phivses of the transition from sound, fresh rock to arable soil in a 

 single outcrop, where all danger of admixture of foreign material is 

 reduced to a minimum, and, second, to the equally interesting if not 

 imi)ortant fact tliat the time-limit of such disintegration can be drawn 

 with a considerable degree of accuracy. The investigations here detailed 

 were undertaken with a view to ascertain, so far as i)08sible. both the 



XLV— Bli.i.. Geol. Sor. Am.. Vol. C, 1894. (321) 



