THE WEATHERING OF EOCKS (Continued) 



II. CONSIDERATION OF SPECIAL CASES 



Let us now enter into a consideration of the composition of 

 a few prominent rock types, and note tlie changes they have 

 undergone in this process of weathering, assuming, as we must 

 for the time being, that they hare been all subjected to essen- 

 tially the same conditions. Inasmuch as there are divers types 

 of rocks, differing not merely in chemical and mineral composi- 

 tion, but in structure as well, it is an easy assumption that the 

 results of prolonged weathering may be widely divergent. Tet, 

 as will become apparent, the ultimate products from all but the 

 purely quartzose rocks, present striking similarities. 



Weathering of Granite. — In the tables following are given 

 the results of chemical and mechanical analyses of rocks of 

 various kinds and in varying stages of degeneration. We will 

 begin with a consideration of the granitic rocks of the District 

 of Columbia.^ The climate of the region, it should be stated, is 

 somewhat capricious, the Weather Bureau records showing ex- 

 treme ranges of — 15° to + 104° Fahr., while an annual range 

 from 10° to 95° is common. The average annual temperature is 

 54.7° Fahr., and the average precipitation 43.96 inches. 



The rock in its fresh condition is a strongly foliated gray 

 micaceous granite showing to the unaided eye a finely gran- 

 ular aggregate of quartz and feldspars arranged in imperfect 

 lenticular masses from 2 to 5 mm, in diameter, about and 

 through which are distributed abundant folia of black mica. 

 In the thin section the structure is seen to be eataclastic. 

 Quartz and black mica are the most prominent constituents, 

 though there are abundant feldspars of both potash and soda- 

 lime varieties, which, owing to their limpidity, can by the 

 unaided eye scarcely be distinguished from the quartz. The 

 potash feldspar has in part a microcline structure. Aside from 

 these minerals, a primary epidote, in small granules and at times 



^Disintegration of the Granitic Eocks of tlie District of Columbia, Bull. 

 Gaol. Soc. of America, Vol. YI, 1895, pp. 321, 332. 



186 



