GEOLOGICAL AND MINERALOGICAL NOTES. 91 



correct ; still it might have been found in boulders, which 

 are produced by chemicals and atmospheric disintegration 

 as oxydation of iron, solution of the salts, alteration of 

 the hornblende, or hydration of the feldspar. Everywhere 

 on the Neck this process may be observed. Even whole 

 ledges of syenite and diorite, with diabase dykes cutting 

 through them, are seen reduced to decayed rock. Change 

 of temperature from hot to cold causes expansion and con- 

 traction. This cracks the rocks, thus letting in rain-water, 

 charged with carbonic acid, which causes disintegration 

 and a gradual rounding of the ruck-masses. The granite 

 boulders of Peabody are actually standing upon other boul- 

 ders of the same kind of granite, which are covered by 

 the rotted granite, which forms the soil over and around 

 them. This is conclusive proof that their origin is from 

 disintegration in situ and not from glacial deposits from 

 a distance, also that the so-called glaciated surfaces under 

 them are merely fragments of slickensides. A visit to any 

 of the granite quarries either in Peabody or Rockport will 

 prove the presence of these slickensides at each joint plane 

 at various depths from one to sixty feet. Some of the 

 deepest and most extensive quarries in Peabody are in mas- 

 sive boulders which show in outline the disintegration of 

 the joint planes. In the comparison of joint structures 

 between the Peabody and Rockport quarries we may form 

 some idea of the amount of erosion that has taken place 

 at Peabody and vicinity. At Rockport the upper joints are 

 from three inches to one foot thick and at sixty feet deep 

 in the quarries they are from fifteen to twenty feet thick ; 

 while in the Peabody quarries the upper joints are from 

 twelve to fifteen feet thick and the second joint is often 

 twenty-five feet thick. Now if this granite is part of one 

 continuous sheet across Peabody to Rockport the amount 

 of erosion at Peabody must be from sixty to seventy feet, 



