53 



Returning now to the question of the geographic separation 

 of the Huronian petrosilex and the petrosilicious rocks of the 

 Shawmut group, it is hardly necessary to observe that the dis- 

 tinctness and simplicity of their common boundaries, as repre- 

 sented on the map , are in most cases a very inadequate expres- 

 sion of the complexity observable in the field. Along the 

 River Parker, in Newbury, the Shawmut rocks are non-petro- 

 silicious, or, when petrosilicious, so far as I have observed, 

 they are never compact, but have a distinct breccia structure ; 

 and hence they may be readily distinguished from the adjoining 

 petrosilex. On Marblehead Neck there are several limited 

 areas of breccia. These are mere superficial patches lying ir- 

 regularly upon the petrosilex ; and they are much smaller and 

 more numerous than indicated on the map. They are repre- 

 sented as occurring chiefly along the shore ; but it is not im- 

 probable that more or less breccia exists in the central portion 

 of the Neck, covered by the soil. The islands north-east of 

 Marblehead Neck marked on the map as composed wholly of 

 petrosilex, nearly all comprise more or less Shawmut rock. It 

 is represented as occurring on Lowell's Island, and I 'have also 

 observed it on Marblehead Rock, Half- Way Rock, and South 

 Gooseberry Island. The Shawmut breccia appears at Red Rock 

 on the Lynn shore, but does not, apparently, extend far inland. 

 The petrosilex in the immediate vicinity of Dungeon Rock, in 

 Lynn, is distinctly stratified, and may belong to the Shawmut 

 group ; yet I think it is more probably Huronian. As already 

 indicated, the breccia occurs at many points, but not continu- 

 ously, through southern Saugus ; and I have observed it as far 

 north as the Pirate's Glen, where the pebbles are small and the 

 rock more compact than south of the railroad. In Maiden, 

 east of Maplewood, the petrosilex is skirted at intervals along 

 its southern border, following Salem Street, by breccia. This 

 breccia is also less distinct, and more like petrosilex, than that 

 in the vicinity of Cliftondale and East Saugus. Following the 

 road north from Maplewood, we find nothing but breccia be- 

 tween the Saugus Branch R.R. and the granite in Melrose. 



