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many points ; but nowhere is it so well developed as in the 

 vicinity of Castle Rock. Here the fragments are of all sizes 

 up to two inches across, and represent every type of petrosilex 

 and felsite occurring in the region. The larger fragments, 

 especially, are usually quite angular, and none, so far as I 

 have observed, exhibit any traces of distortion ; but, on the 

 contrary, as already stated, this breccia rests upon, and has 

 been cut after the manner of an exotic by, the schistose or 

 " flattened pebble " petrosilex, fragments of which are included 

 among the pebbles of the breccia. Being simply the very 

 thoroughly reconsolidated debris of petrosilex and felsite, the 

 breccia, especially the more compact varieties, is often practi- 

 cally indistinguishable by the unaided vision from the parent 

 rocks. Between Castle Rock and the north end of the Neck 

 there is a large mass of rock which is decidedly porphyritic, 

 and holds but few distinct pebbles ; and yet, notwithstanding 

 its compact texture and ancient aspect, it is certainly newer 

 than the banded petrosilex on the north, and it is met and dis- 

 tinctly cut by eruptive, crystalline, grayish and black petrosilex 

 on the south. A similar rock, but containing much free quartz, 

 occurs on the north end of the Neck, and at other points. As 

 a rule the compact feldspathic rocks of the Shawmut group 

 have the waxy lustre and inferior hardness of felsite, rarely re- 

 sembling true petrosilex. 



Shawmut Group in the Lynn and Medford Region. — As 

 in the Marblehead district, the Shawmut group in the large area 

 between eastern Lynn and Medford, and between the south 

 part of Maiden and Wakefield, is, so far as I know, represented 

 by the breccia division alone ; with the single exception of the, 

 in part, well-marked and beautiful amygdaloid found in Saugus 

 Centre, on a small conical hill between the Saugus River and 

 the railroad. This is evidently of very limited extent ; but its 

 boundaries and relations cannot be observed on account of the 

 drift. The base of the rock is of a dull green color and slaty 

 texture ; the amygdaloidal kernels are numerous, small, have a 

 radiate structure, and are composed of feldspar. Red Rock, on 



