WEATHERTNa INPLUENOED BY STEUCTURE 



231 



joint blocks, and has been argued by some of the earlier geologists 

 as indicative of an original concretionary structure. Such an 

 assumption seems, however, wholly uncalled for. If the block or 

 mass is reasonably homogeneous, the agencies of decomposition 

 will penetrate nearly uniformly from all equally exposed sur- 

 faces, producing an exfoliation nearly parallel to that surface, 

 and the concentric structure is inevitable, as was long ago 

 pointed out. 



In some cases the tendency to assume the boss-like form is 

 accentuated through the presence of joints running approxi- 

 mately parallel to the exposed surface, such joints as give rise> 

 to the step-like arrangement of the stone so frequently seen in 

 granite quarries. Stone Mountain, Georgia, an immense boss 

 of light gray granite some 2 miles long by 1| wide and 650 feet 



Fig. 18. — ^Exfoliation of granite. 



high, owes its form, apparently, wholly to exfoliation parallel 

 to pre-existing lines of weakness. The entire mass, so far as 

 exposed by quarrying operations, is made up of imbricated sheets 

 of granite, which, of unknown thickness beneath the surface, 

 thin out to mere knife edges above, like shingles on a roof. 

 Through prolonged exposure the superficial layers have become 

 detached from the parent mass, and doubtless hundreds of feet 

 in vertical thickness completely disintegrated and swept away. 

 "With many geologists these joints, in themselves, would be ac- 

 cepted as due to atmospheric action. The boss-like form is there- 

 fore incidental and consequent. The process of exfoliation has, 

 in the case mentioned, been productive of some peculiar results 

 which may be described in detail. 



