200 



a«lvent of an epocli of severe coniprcssion of the eartli's crust 

 ill this region, in consequence of whicli the sedimentary rocks 

 ^vere elevated to form dry land, folded, faulted, and injected 

 by dikes of diabase. During all tlie long ages since this 

 geographical revolution, Hinghani has been mainly, if not 

 continuously, a land area ; and the slate, together with the 

 underlying conglomerate, has suffered enormous erosion. 

 These rocks have thus been completely removed from large 

 areas of granite which they once covered ; and they are 

 preserved to ns now only where they were most deeply 

 folded or faulted down between the granitic masses, and thus 

 protected from erosion. 



GENERAL STRUCTURE OF NORTHERX IIINGIIAM. 



The eastern shore of Iliiigham Harbor is not only the 

 natural boundary line between tlie geological districts of 

 Nantasket and northern Ilingham, but it jn'obably marks 

 the position, as shown on the general map, of one of the 

 great transverse faults of the Soutli Sliore ; and it certainly 

 corres[)onds, as already explained, to a very lm[)ortaiit con- 

 trast in geological structure, plication being as characteristic 

 of the Hingham area as faulting is of the Nantasket area. 

 As the general map so clearly shows, the key to the structure 

 of the volcanic and sedimentary rocks of Ilingham is the 

 oblong area of granite and felsite lying north of the raih"oad 

 and Beal Street, and bearing the three drumlins of Bradley's, 

 S(piirrel, and Baker's Hills. The general position of this 

 mass is unquestiona])ly anticlinal. This is most obvious at 

 the western extremity, where the melaphyr and the sedi- 

 mentary strata curve around the granite and dip away from 

 it on both sides. Southward from this point, between Beal 

 Street and Beal's Cove, the structure is monoclinal ; and the 

 ledges aff(n'd a nearly continuous section across the entire 

 consflomerate series and a considerable thickness of the over- 



