10 GUIDE TO LOCALITIES. 



in the rocky portion of the coast, clue to primary or secondary 

 structures, dikes, etc. The protective effects of barnacles and 

 seaweeds can be studied successfully. 



On the east side of the shore road, a few hundred yards north 

 of Clifton station, is a striated ledge of granite, with its lee side 

 toward the east. This side presents the typical curves of marine 

 erosion, accomplished apparently after most of the glaciation. 

 The whole was buried in stratified drift, and has been exposed 

 only recently through road building. The surface of the ledge is 

 about fifty feet above the sea. 



The region possesses great petrographic interest. From about 

 the middle of Swampscott northeast the rocks have the same gen- 

 eral characters almost to Marblehead Neck. The odest is diorite, 

 into which at an early stage a granitite has intruded. The relations 

 between the two are various. In general, the contacts are dim 

 and intricate. At one place the older rock prevails, at another the 

 younger; and in many ledges, the mass is a breccia of granitite 

 carrying irregular horses of diorite. Into these rocks a lighter 

 granitite and an eleolite syenite have penetrated, and lastly a black 

 trap. This is largely in dikes which run roughly parallel to the 

 coast, and have well defined walls, often separated from the country 

 rock. Since the last intrusions, which are probably Mesozoic, a 

 great amount of faulting has occurred, much of it in a southeast 

 direction. The " Lincoln dike," off Clifton, is the largest and best- 

 known case. In the trap basaltic structure is roughly developed 

 in some cases, and many dikes have porphyritic centers. 



Farther north, including the southern part of Marblehead Neck, 

 the main rock is porphyrite. Beyond here it is a coarse granitite 

 for the most part, on the ocean side. The well-known Bostonite 

 occurs on the west side of the neck, but can be found only at 

 low tide, and by diligent search. 



Literature. 



Sears, J. H. — Various articles on the geology of Essex county, in 

 Essex Inst. Bull. 



Route, — By rail, Eastern division Boston and Maine road, from 

 North Union station, to Lynn; fare, $.L'0. 



By electrics, Lynn and Boston road, Scollay square to Lynn; round 

 trip fare, $.25. 



