24 GUIDE TO LOCALITIES. 



composite lava flow. This melaphyr should probably be corre- 

 lated with the Second or Third Melaphyr east of the railroad. 



Northeast of Cliff plateau, the section of West Porphyrite hill 

 is exactly repeated in East Porphyrite hill, except that we find 

 above the porphyrite bed traces of the Third Conglomerate, which 

 has a more complete development immediately to the south, in 

 Conglomerate hill. Great hill shows granite on the south overlain 

 by the basal conglomerate and First Melaphyr ; but these are cut 

 off abruptly by a fault which lets clown the Third Conglomerate 

 to form the main body of the hill. 



Southwest of Great hill, on the border of the marsh, the granite 

 is cut by a large dike of melaphyr identical with the First Mela- 

 phyr and believed to be the vent through which that flow reached 

 the surface. The diabase dikes are similar to those of the coastal 

 area, two prominent systems of east-west dikes being cut by a 

 north-south system ; and the finest example of the latter is a com- 

 posite dike on the shore between Cliff plateau and East Porphyrite 

 hill. 



Rocky neck, in Hingham, on the west side of Weir river, is geo- 

 logically very similar to the western area of Nautasket, but it is 

 less accessible. 



3. The Cohasset shore. 



This excursion embraces the shore ledges along Jerusalem road 

 from near Green hill, Nantasket, to Cohasset. 



Boute. — The best route is by boat or rail to Nantasket station, and 

 thence by electric cars to Jerusalem road near the Black Rock house. 

 The phenomena of most particular interest may be observed between 

 Green Hill beach and Pleasant beach (one and a half miles), whence the 

 return can be made on foot or by barge back to the Black Rock house and 

 Nantasket; or the walk may be continued to a point just beyond the 

 bridge across the mouth of Little harbor and thence by the right hand 

 road to Cohasset village (four miles in all), returning by train to Boston. 



No locality near Boston offers a finer field for the study of plu- 

 tonic rocks and dikes. The former embrace: (1) the normal 

 biotite granite (granitite) ; (2) the basic or dioritic granite, pass- 

 ing into (3) diorite ; and (4) fine-grained and highly acid granite, 

 passing into micro-granite and quartz porphyry. 



Numbers 1 and 2, the latter especially, are in part distinctly 

 gneissoid (flow structure). Number 1 cuts 2, and both cut and 



