72 



From this petrosilex, which can hardly be regarded as a 

 distinct variety, we pass to the much more abundant rock in 

 which feldspar crystals are numerous and conspicuous, — atyp- 

 ical porphyry according to the old nomenclature. As before, 

 the base is compact and structureless ; its colors are black, gray, 

 and purplish. The largest mass of this rock, perhaps, is on 

 the seaward side of the Neck, immediately south of the beach. 

 At this point the base has a finely granular aspect, and the 

 feldspar crystals are large and numerous, and in addition there 

 are many small roundish masses of epidote. The rock is tough 

 and does not break with a smooth fracture. The general tint 

 is dark purple, but the weathered surface is white ; and it is said 

 that from these bold cliffs of white petrosilex the town of Mar- 

 blehead derives its name. At the contact of this petrosilex 

 with the granite on the south there are two alternations of petro- 

 silex and granite within a distance of four or five rods, and it is 

 clear that one of these rocks has played the role of an eruptive. 

 There appears to be a considerable area of this coarsely por- 

 phyritic, purplish petrosilex in the central portion of the Neck. 

 On Marblehead Rock and the small island east of Lowell's 

 Island the base is black. At the first locality the rock is 

 heavy and trappean, and unquestionably eruptive through the 

 Shawmut slate and breccia occurring there ; while at the second 

 point the aspect is much the same, but the rock is more por- 

 phyritic. A fair specimen from this small island contains 

 only 69 per cent, of silica, hardly enough for genuine petro- 

 silex ; and the Marblehead Rock exotic, although it has not 

 been analyzed, is certainly still more basic, and may be set 

 down as a true felsite. 



In the southern part of Marblehead Neck there is a gradual 

 transition from this normally porphyritic type of petrosilex to 

 a rock in which the compact base is wanting, or nearly so, being 

 crystalline throughout. This variety is of an ashen-gray color, 

 weathering a dull white. I have observed it principally in the 

 immediate vicinity of the granite toward the south-western end 

 of the Neck, yet it forms a large mass on the ocean side, mid- 



