170 



the Lynn shore, is composed mainly, if not wholly, of breccia. 

 There are two distinct colors, brownish-red and gray. , A 

 small mass on the west side of the point has the aspect of 

 conglomerate. Breccia is known to occur at many points in 

 the west part of Lynn ; but the areas have not been indicated 

 on the map. The breccia of East Saugus, Cliftondale, and 

 Franklin Park has been described, and the relations to the 

 underlying petrosilex noted (ante, p. 52). The large areas 

 marked as breccia in Wakefield, Melrose, and Maiden, have 

 been only very imperfectly explored, and it is not unlikely that 

 they include considerable petrosilex. There are many types 

 of breccia here, but they agree very well with those of Marble- 

 head Neck, and are similarly related to the petrosilex. Some 

 of the breccia in Maiden and Melrose is much coarser than any 

 that occurs in the Marblehead region ; and the base is some- 

 times more porphyritic, though in other cases it is quite 

 compact. Occasionally there is an obvious tendency to de- 

 velop a schistose structure similar to that characterizing a 

 portion of the petrosilex of this and other areas, but it never 

 passes the incipient stage ; and it can throw no doubt on the 

 chronological distinctness of the petrosilex and breccia, be- 

 cause, as already noted (p. 75), fragments of the schistose 

 petrosilex are included in this same breccia. Although derived 

 mainly from the different types of petrosilex occurring in those 

 towns, yet the breccia of Maiden, Melrose, etc., holds occa- 

 sional pebbles of granite and diorite. Magnetite is of rare 

 occurrence in these rocks. The wedge-shaped area marked as 

 breccia in West Medford includes no true breccia, but is 

 composed of a fine-grained arenaceous and slaty rock, which 

 resembles at different points quartzite, eurite, and felsite. 

 The stratification is sufficiently obvious, and I have no doubt 

 that it is a recomposed rock. 



Shawmut Group in Brighton, Newton, and Need- 

 ham. — We come once more to a district characterized by 

 the Shawmut group, in which amygdaloid is the prevailing 

 type. The large and irregular area of the rocks of this age 



