?-,: 



74 



Causes of Geological Change 



these slides took place in 1784 ; and that it swept away 

 one dwelling house^ though the inmates escaped. 



POWER OF ICE IN REMOVING BOULDERS FROM PONDS. 



It is well known that w^ater, by an apparent exception 

 to a general law, expands with great force when freezing, 

 and even for several degrees above and below the freez- 

 ing point. Over a large surface this effect may be con- 

 siderable ; and when boulder stones, lying in shallow 

 ponas, become partially enveloped in ice, they must feel 

 the effect of this expansion, and be forced towards the 

 shore ; since the expansive force must always act In that 

 direction, 

 rock 



As no antagonist force exists to drive the 



back again to its original 



position, the ultimate 

 result of the freezing, during successive winters, must be, 

 to crowd it entirely out of the pond. And may we not 

 in this way explain the fact, that sometimes we see the 

 margin of a pond lined with a ridge of boulders, while the 

 bottom is comparatively iree from them ? 



This curious fact has yet been noticed in but a few 

 places in New England, probably because the attention 

 of observing men has not been directed to it. 



ENCROACHMENT OF THE SEA UPON THE LAND. 



Boston harbor furnishes a striking example of this kind 

 of action- The numerous islands in this harbor as far 

 outward as the Great Brewster, consist chiefly of diluvial 

 sand and gravel, resting upon slaty, slenltic, and con- 

 glomerate rocks. Beyond the Great Brewster, however, 

 this diluvium is wanting ; and the islands consist almost 

 entirely of naked rock. It would be reasonable to Infer 



A^ 



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