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ending in a thud that makes the earth tremble, followed by 

 sheets of dashing spray, heightens the sublimity of the 

 scene. The structure of the Greenstone rock which consti- 

 tute these dikes, some of which are three feet in width, being- 

 somewhat columnar and the line of direction being at right 

 angles Avith the coastline, facilitates the excavating action 

 of the waves. The vitrified sides of these Purgatories are 

 a striking proof to the young geologist that their contents 

 were once fused. Out he surface of a dike, in a large, isola- 

 ted bluff of Syenite forming one side of a sandy beech on the 

 ocean-side of the Neck, nearly opposite the north extremity 

 of Tinker's Island, may be found a fragment of the Syenite 

 ledge weighing two or three pounds, about half immersed in 

 the Greenstone at the surface of the dike, and within a few 

 inches may be seen its original place in the ledge. I have 

 never met Avith a more conclusive proof than this that the 

 contents of these dikes came up in a fused state after the 

 Syenite had cooled and become solid. The northern sides of 

 the hills of Marblchead, (and all her hills are ledges,) are 

 abrupt while upon the surface of the formations, whenever 

 laid bare, may be readily found a rounded surface abounding 

 with scratches, that are readily distinguished when the 

 sun is near the horizon, evidences of drift action. 



In the northern section of the town but little drift is found, 

 which in the southern section begins the great drift plain, 

 extending, with varying width, a distance of about two 

 miles. The substrata of the plain varies in different portions, 

 but is mostly of a rather coarse gravel, though some portions, 

 rest on deposits of sand. On a ledge which outcrops on 

 High street, presenting a level surface, near the residence 

 of Mr. Thomas Swasey, may be seen several fine illustrations 

 of what I must assume to he marks of glacial action. At 

 the southern extremity of the township, near the Salem road, 

 not far from the Rose farm, are vast natural depressions in 

 the earth, popularly known as "The Dungeon." The lar- 

 gest of these is almost a perfect oval, from two to three hun- 

 dred feet across and of great depth. The only theory that 

 will plausibly explain their origin, is, that which supposes 

 them to indicate the places where some of the icebergs of the 

 drift period grounded, the deluge of water still having power 

 to bear along the gravel in its course, which filled in around 

 the grounded and rounded icebergs ; in the course of time 

 these melted and left their moulds in the depressions that 

 now meet the eye. 



