A SKETCH OF THE GEOLOGY AND MINERALOGY OP THE SOUTH- 

 ERN PART OF ESSEX COUNTY, IN MASSACHUSETTS. COMMUNI- 

 CATED TO THE ESSEX COUNTY NATURAL HISTORY SOCIETY, 

 APRIL 24, 1839. BY WM. PRESCOTT. 



THE following sketch embraces the city of Salem and the 

 towns of Beverly, Danvers, Marblehead, Lynn, Nahant,* Sau- 

 gus and Lynnfield. The principal rocks, areSienite, Greenstone, 

 Porphyry, Silicious Breccia arid Brecciated Porphyry, Chlorite 

 Slate, Argillaceous Slate, Silicious Slate, Magnesian or Verd 

 d'Antique Marble, and Varioloid Wacke. Sienite and Green- 

 stone are by far the most abundant rocks. 



GRANITE. 



No bed or deposit of granite occurs in this section of Essex 

 County. A few scattering bowlders only are found in Lynn and 

 Saugus, which increase in size and frequency as you proceed 

 north, through Danvers and Lynnfield, towards the beds of 

 Granite in Boxford. These bowlders of granite are of a very 

 dark color from the black mica which they contain, and in the 

 north part of Danvers, frequently occur several feet in diameter. 



Bowlders and masses of a light colored granite are frequently 

 seen cast upon the shore, on the east side of Nahant, and on 

 Lynn beaches. 



GNEISS. 



The rocks at the north west part of Danvers are more or less 

 stratified, evidently the termination of the Gneiss and Mica slate 

 formations. This structure may be seen as far as Newburyport 

 turnpike, and occurs rarely south east of that road. 



* Notwithstanding Nahant constitutes a part of the town of Lynn, yet, isolated, as it is, 

 from the main land, stretching several miles into Massachusetts Bay ; and being in many 

 respects peculiar in its geological features, it will in the following sketch, be made a locality 

 for distinct reference. For particular localities, the reader is referred to Lewis' map of 

 Lynn and Saugus. 



