63 



that have come under my notice where the concretionary or 

 pearlstone structure in the petrosilex is well marked ; but indi- 

 cations are not wanting that this structure, or a tendency to- 

 ward it, is very general in this rock. 



Petrosilex on Marblehead JSTeck and the adjacent islands. 

 — There are several distinct varieties of petrosilex on Marble- 

 head Neck. The most conspicuous and beautiful of these is the 

 banded variety already alluded to. This has substantially the 

 same characters as in Newbury. The bands, however, are gener- 

 ally broader, — one-sixteenth to one-eighth, and sometimes one- 

 fourth of an inch. The feldspathic layers are rarely reddish, 

 usually gray, bluish, or purplish, but always weathering whitish. 

 The banding is often very indistinct on a fresh surface. The 

 bands are, perhaps, less regular and continuous than in the 

 Newbury rock, coalescing and dividing more frequently. 

 They exhibit greater disturbance, and at many points are 

 locally resolved into a condition approaching the chaotic, 

 portions of the rock appearing to have been crushed and 

 partially fused : yet, on the whole, a north-easterly strike 

 appears to prevail, and this is no less clearly a stratified 

 rock. Crystals of feldspar are common in this rock, but are 

 rarely numerous enough to give it a porphyritic aspect. This 

 variety of petrosilex is well developed on the north-west side 

 of the Neck, and in the vicinity of the light-house at the 

 northern end. Several small patches also occur on the east- 

 ern shore near Castle Rock, and to the northward. A typical 

 specimen from near the southern border of the petrosilex, on 

 the harbor side of the Neck, contains 77.5 per cent, of silica. 

 This indicates probably thirty or forty per cent, of free quartz, 

 and shows that the rock must be regarded as a true petrosilex : 

 a conclusion which might have been inferred with hardly less 

 certainty from its great hardness (above 6) and its jaspery 

 fracture. This is a much-jointed' rock. At some points, 

 especially in the small patches on the eastern shore, the banded 

 petrosilex where most disturbed passes into an unstratified 

 variety, which becomes porphyritic with feldspar crystals, and 



