1884.] 15 [Crosby. 



islands, of the crystalline rocks, such as occur in Milton, Hyde 

 Park, etc., are usually entirely surrounded by a zone of conglom- 

 erate. 



(2) The conglomerate almost invariably dips toward the out- 

 crops of slate, and in not a few instances can actually be seen 

 to pass beneath that rock. The reverse of this sometimes 

 occurs, as in the Beacon Street section near Newton Centre ; but 

 this is most naturally explained as due to an inversion of the beds 

 — an explanation which, however, will not apply in the far more 

 numerous cases where the conglomerate is the underlying rock. 

 Following is a list of the localities where the superior or synclinal 

 position of the slate is clearly shown : — Several points in the 

 northwest part of Hingham, as described in my " Contribu- 

 tions ; " vicinity of Black's Creek, in Quincy, the slate south of 

 the conglomerate dipping toward the south, and that north of the 

 conglomerate dipping toward the north ; several of the patches 

 of slate in the valley of the Neponset ; North Quincy, one-half 

 mile northeast of Atlantic Station ; all along the Dorchester and 

 West Roxbury belt of slate, but especially in and near the Mt. 

 Hope and Forest Hills Cemeteries; in the vicinity of Newton 

 Centre, and Chestnut Hill Resevoir ; Newton Corner and South 

 Natick ; and the square in Watertown bounded by Mt. Auburn, 

 School, Belmont, and Grove Streets. 



In short, save where occurring on the margins of the basin, or 

 exposed through the agency of faults, the conglomerate never 

 comes to the surface except through denuded anticlinal s from 

 which the slate has been eroded away ; though in some cases the 

 conglomerate too has been worn through, bringing into view the 

 crystalline axis beneath. 



The supposed occurrence of pebbles of the slate in the con- 

 glomerate has been confidently appealed to as conclusively prov- 

 ing the greater antiquity of a portion at least of the former 

 rock ; but after a critical examination of many hundreds of these 

 " slate pebbles," I am fully convinced that I have not seen a sin- 

 gle genuine pebble that can be clearly identified with any part of 

 the slate formation now exposed to observation. Many seeming 

 slate pebbles are merely very limited, lenticular layers of slate 

 intercalated in the conglomerate. Good examples of such false 

 pebbles resulting from irregular sedimentation occur in North 



