of the Southern part of Essex County. 8 1 



In some portions of it, the feldspar is compact, and the mass 

 appears almost homogeneous. 



GREENSTONE, 



Greenstone trap is the most abundant rock in this section. It 

 prevails in Marblehead, Salem, all that part of Danvers north 

 and east of Proctor's Brook, the whole of Beverly, with the ex- 

 ception of the bd of sienite before described, and the east part 

 of Lynn. Greenstone may be seen also, protruding above the 

 surface, near the base of all the faiils of porphyry, in Lynn and 

 Saugus, and also, at the base of the hills and ledges of sienite^ 

 in Danvers, Lynn, Saugus and Beverly. Greenstone, occurs 

 on the west side of Nahant, including Black Rock, and also on 

 the east part of Little Nahant. 



Dykes of a fine grained compact greenstone or trap, from one 

 inch to forty feet in diameter, traverse the slate and sienite in 

 every section of Nahant and Beverly, as well as the coarser 

 variety of greenstone in Salem and Marblehead. 



The greenstone, in this region, presents a considerable diver- 

 sity of aspect, depending on the general structure, or on the 

 size, proportion and mixture of its constituent parts. In some 

 of the more common varieties, the two ingredients are in distinct 

 grains of considerable size, like those of granite, aud sometimes 

 the grains are so minute, and SQ intimately and uniformly min- 

 gled, that the mass appears altogether homogeneous. 



The chief part of the greenstone, especially in Salem and 

 Marblehead, is injected by numerous veins of sienite, or rather 

 of quartz and feldspar, the hornblende generally being nearly or 

 quite wanting. These veins, which traverse the greenstone in 

 every direction, are, in many places so numerous as to give the 

 rock quite a brecciated appearance. This appearance is very 

 conspicuous at the south end of Beverly bridge ; along the sea- 

 shore at Marblehead, and various other places, where the rocks 

 have been exposed by blasting for the Eastern Rail Road, and 

 for other purposes. 



Many of the trap dykes are rendered porphyritic, which, by 

 contrasting the crystals of white feldspar, with the black horn- 

 blende of the trap, becomes quite beautiful when polished. 



Greenstone porphyry, and compact feldspar porphyry passing 



