geology: south shore. 25 



enclose the diorite, in which the gneissoid structure also often is 

 strongly marked. The fine granite (4) forms narrow branching 

 dikes, cutting all the other types. All are regarded as successive 

 phases developed during the cooling and crystallization of one 

 magma ; and the diorite is believed to occur largely but not ex- 

 clusively as segregations in the basic granite. 



The dikes embrace, in order of age: (1) porphyrite dikes, 

 which have a general north-south trend and are believed to radiate 

 from the great mass of porphyrite forming the Black Rock islet and 

 probably marking the vent for this type of lava ; (2) the three 

 systems of diabase dikes noted at Nantasket. Some very clear 

 and interesting intersections are exposed. If this excursion is 

 extended to Cohasset village, an hour should be resei-ved for a 

 visit to the glacial pot holes on Cooper's island (really a peninsula) 

 in Little harbor. The beautiful sloping sand-plain on which the 

 village stands, with its bold ice-margin toward Little harbor, and 

 fine kettle holes, one of which is called the Punch-bowl, also 

 should be noted. 



Literature. 



Crosby, W. O.— Geology of the Boston Basin. (Bos. Soc. Nat. Hist., 

 Occasional Papers, vol. iv, part 1, 1893.) 



NORTHERN HINGHAM. 



iJoM<€. — This district may be reached by the Nantasket steamers to 

 Downer Landing and Hingham, or by the South Sliore trains of the Old 

 Colony railroad from the Kneeland street station. 



The areas of sedimentary rocks in the northern part of Hingham 

 present several good sections of the Carboniferous strata of the 

 Boston Basin, contrasting with and supplementing the Nantasket 

 sections. The volcanic rocks are less abundant ; but there is 

 above the granitic rocks one heavy bed of melaphyr followed by 

 many alternations of conglomerate and red slate, and this conglom- 

 erate series is conformably overlain by a great thickness of gray 

 slate, while plication takes the place of ifaulting as the dominant 

 type of disturbance or secondary structure. 



The most accessible section is that at Downer Landing, in Mel- 

 ville Garden and adjoining fields, and on the islands of Hingham 

 harbor (row boat from Melville Garden), a gracefully curving 

 monocline of alternating conglomerates and finer sediments, with 

 several very obvious transverse faults. 



I 



