51 



sketch taken on the spot. These are connected by 

 ground shades of the same color, but lighter hue, which 

 indicate to the experienced eye the character of the rock 

 lying under the soil. Thus the theoretical conclusions 

 are separable from the observed facts, and every person 

 can see the face of the country as it is, the conventional 

 signs and colors being readily translated by the table at- 

 tached to the sheet. 



Marblehead Xeck was selected as the initial point, be- 

 cause it is one of the three porphyry outcrops within the 

 limits of this county, and from these it is convenient to 

 lay our courses in order to meet the regularly stratified 

 rocks which lie to the westward. 



It would be premature at this stage of the investiga- 

 tion to endeavor to forestall the results which may be 

 finally worked out, but it is possible to make certain 

 statements of considerable interest. 



The porphyry of our vicinity whether Lynn, Marble- 

 head or Xewburyport is a recomposed rock, a conglom- 

 erate composed of more or less rounded pebbles of more 

 ancient banded porphyry. Taking our departure from 

 these points we meet in the neighborhood of Xewbury- 

 port with a transition rock made up partly of porphyry, 

 and then with stratified diorites and slates, which surround 

 the porphyry outcrop on the sea-face, and stretch up to 

 the north and south of Kent's Island, and are lost in the 

 marshes. 



The northwesterly dip, and northeasterly strike of 

 these diorites and slates, and the presence of slate rocks 

 in Topsfield and Middleton, are difficult to account for 

 unless we imagine the porphyries to be interstratified 

 with them. The succession of the strata in this part of 

 the county then would be Eozoonal limestones and serpen- 

 tines, then slates, then the porphyries of Kent's Island 



!/ 



