78 



a rule, occasionally assumes the form of slender crystals half an 

 inch long. Grains of quartz are rare, and usually wanting. 

 Some of the feldspar, at least, is triclinic ; and, although marked 

 on the map as petrosilicious, this rock, in the vicinity of Fulton 

 Street, is more properly a diorite. To the eastward, how- 

 ever, its physical characters change ; and near the line between 

 Medford and Melrose it appears through the Shawmut brec- 

 cia as a compact gray base, holding conspicuous crystals of 

 plagioclastic feldspar and slender needles of hornblende. Thus 

 it is clear that we here have a felsite passing into diorite. A 

 somewhat similar, but more porphyritic, rock is found still far- 

 ther east, beyond the Boston and Maine Railroad. This closely 

 resembles the crystalline layers in the stratified felsite at Dun- 

 geon Rock in Lynn, and, like that, it has the composition of a 

 true felsite, the percentage of silica being not quite 63. It car- 

 ries a few pebbles, however, and hence I am obliged to regard 

 it as possibly a recomposed rock, related to the diorite only by 

 derivation. Near the northern boundary of Melrose it appears 

 again, decidedly crystalline and free from pebbles ; and on the 

 west side of the railroad at Greenwood Station there is an im- 

 mense exposure of a light-colored, feldspathic, crystalline rock 

 resembling that on Fulton Street in Medford, but finer grained, 

 which should be set down as diorite, though probably passing 

 into felsite. It is very basic, yielding only 57 per cent, of silica. 

 Indications are not wanting that all of these dioritic felsites in 

 Melrose and Wakefield may belong to the Shawmut group. 



Granitoid petrosilex, such as occurs on the south-western end 

 of Marblehead Neck, has been observed at only two points in 

 the Lynn and Medford area, on the Newburyport turnpike in 

 Maiden, and in North Saugus, near where Main street crosses 

 the Wakefield line. At both places the rock is very local, and 

 yet its transition character is plain. 



This extended area of petrosilicious rocks, although many 

 times larger than the Marblehead Neck region, and fully equal- 

 ling that limited area in structural complexity and the variety 

 of problems which it presents, has been the field of even fewer 



