82 PrescolCs Sketch of the Geology and Mineralogy 



into greenstone porphyry, some of which contains crystals, of 

 a beautiful variety of green feldspar, occurs at Nahant and sev- 

 eral places in Lynn, as will be more fully explained under the 

 head of porphyry. 



PORPHYRY, 



The only range of porphyry, extends from the village of Wood 

 End in Lynn to Chelsea, through Lynn and Saugus. This 

 range is bounded on the north, by the range of sienite before de- 

 scribed, and on the south, by tide water, varying in breadth from 

 cue to two miles, forming a range of barren hills and craggy 

 cliffs. The base of this porphyry is compact feldspar, its colors 

 various, but generally some shade of red or brown ; but grey, 

 purple and black are not uncommon. Some portion of it is des- 

 titute of crystals, so from this circumstance and its peculiar 

 hardness and fracture, it has generally been considered as a 

 variety of hornstone. 



The eminence called Lover's Leap in Lynn, and a spur of 

 Pine Hill, half a mile west of Lover's Leap, are composed of a 

 solid mass of this species of compact feldspar or hornstone. It 

 also occurs in beds throughout the whole range of porphyry . 

 The most important localities of this variety of hornstone, are 

 Red Rock, at the west end of King's beach, Wood End, vicinity 

 of High Rock, vicinity of India Rubber Manufactory, Lover's 

 Leap, south east spur of Pine Hill, Lynn, besides several places 

 in Saugus. 



Several of the trap veins traversing the trap rocks in the va- 

 rious localities of this formation, are rendered porphyritic by 

 containing distinct crystals of feldspar. There are several of 

 these veins or dykes on Nahant, Marblehead and Beverly, which 

 contain crystals from half an inch to two inches in diameter, 

 some of which are hemitrope. This forms the black porphyry of 

 the ancients. 



The compact feldspar porphyry receives a good polish and 

 will become valuable whenever the wealth and demands of the 

 country shall be sufficient to defray the expenses of quarrying 

 and working it. A peculiar variety of greenstone porphyry oc- 

 curs on Bayley's hill, on the west end of Nahant. The im- 

 bedded crystals are green feldspar, some of which are beautiful. 



