52 



and Lynn, then slates and diorites, and lastly, the por- 

 phyries of Marblehead Neck. Either this is the expla- 

 nation or else we have several anticlinal axes or folds in 

 the porphyry. In either case all the porphyries are 

 probably older than the Eozoonal rocks of Newburyport, 

 and underlie them. The porphyry of Marblehead Neck 

 has the stratified micaceous rocks, mentioned above, lying 

 upon its southeastern face, with dip and strike precisely 

 conformable to the more ancient shore-line formed by the 

 porphyry itself. 



The porphyry of Lynn has upon its eastern face the 

 outcropping edges of an enormous overflow of igneous 

 granite, which anciently filled the valley between Peabody 

 village and Swampscott, but which is now only repre- 

 sented by Prospect Hill and others on the west, and 

 patches which still remain plastered against the western 

 sides of the hills to the east. 



The farther or westerly side of this porphyry, as also 

 of the Newburyport exposure, is occupied by a series of 

 rocks with a regular northwesterly and southeasterly 

 trend. These overlie the series of serpentines and lime- 

 stones which crop gut so abundantly at Newburyport and 

 Lynnfield. At this last locality the relation of these lime 

 rocks to the porphyries is obscured by masses of what I 

 am disposed to consider an eruptive granite. In fact, all 

 the difficulties of the survey have arisen from the enor- 

 mous sheet, or rather, sheets of igneous rocks, for there 

 seems to have been several which overspread the surface 

 of the country. 



The principal seat of one of these eruptions, and per- 

 haps two of them, can be traced to a large part of Mar- 

 blehead and the whole of Salem township. The rock 

 underlying these localities is a dense, dark hornblendic, 

 often micaceous granite, varying in many places to lighter 



