54 



West of the Boston and Maine R.R., in Maiden and Med- 

 ford, the Shawmut petrosilicious rock covers a considerable area. 

 It is a well-marked breccia in the vicinity of Salem Street, but 

 becomes smaller grained toward the north and west ; and near 

 the southern boundaries of Stoneham and Melrose it is often 

 practically indistinguishable, with the naked eye, from Huro- 

 nian petrosilex. The Shawmut rock composing the wedge- 

 shaped area in West Medford, along the north side of the 

 Mystic River, is, in the main, impalpably fine, and easily mistaken 

 for a Huronian rock, yet portions of it have a sandy or slaty 

 texture, and determine the age of the whole. The region 

 between the Maiden Highlands and Maplewood has been but 

 slightly examined. It embraces some petrosilex ; but breccia 

 is probably the prevailing rock. The petrosilicious rocks in 

 northern Saugus and Melrose, and southern Wakefield, belong 

 largely to the newer series. A considerable area of Huronian 

 petrosilex, however, occurs in the vicinity of Crystal Lake, 

 and the Greenwood Station, in Wakefield ; and it crops out at 

 many points in Saugus and Melrose, a few only of which are 

 indicated* on the map. The relative distribution of these two 

 series in this region, — Saugus, Melrose, Maiden, and Medford, 

 - — as shown on the map, can be regarded as accurate only 

 in the most general sense. I have traced the boundaries of 

 none of the areas ; and in some places, especially in North 

 Saugus, the Huronian and Shawmut rocks are so difficult to 

 distinguish lithologically, and are so closely involved petrologi- 

 cally, that it is doubtful if their limits ever can be satisfactorily 

 determined. I believe, however, that I have succeeded in 

 separating roughly the areas in which petrosilex predominates 

 from the areas in which breccia is the prevailing rock. 



The Shawmut rock in Needham, not being petrosilicious, is 

 easily separated from the petrosilex. Although the petrosilex 

 of West Dedham and Dover is without doubt mainly Huronian, 

 I am not confident that it all belongs to that ancient system. 

 The area marked as breccia on the map includes all of that 

 rock and a portion of the petrosilex. The petrosilex west of the 



