The Geology of Hingham, 31 



contain from 56 to 58 per cent of silica. Rocks of this character, 

 of various shades of color, are found at Nantasket, but only one 

 exposure has been noticed in Hingham. This is on the shore of 

 the marsh land that borders Weir River, quite near Rocky Neck. 

 At this place it is of a dark-brown color, similar to that of the 

 brown sandstone commonly used in structures of Boston and 

 New York. It is a heavy, tough rock, and undoubtedly owes its 

 origin to volcanic action, being, like melaphyr, an ancient lava. 



DIABASE. 



Diabase, like Diorite, is composed partly of a triclinic Feldspar, 

 generally Labradorite, but differs from it otherwise in having 

 Augite associated with it instead of Hornblende. Not unfre- 

 quently Mica is found in its composition, and often Pyrite, though 

 these are not essential ingredients. This rock is generally known 

 as Trap, and the dikes which it forms in all parts of the town are 

 called Trap Dikes. The rock as exposed at the surface exhibits 

 more or less the results of decomposition, becoming of a dull 

 green color, from the change of the Augite to Viridite. It has a 

 much higher specific gravity than granite, and is exceedingly 

 tough. The bluish, close-grained masses often found in the soil 

 and called Blue Rocks are of this kind. 



Diabase forms dikes alike in the granitic rocks of the town, 

 and in those of the Slates and Conglomerates to be hereafter 

 mentioned. 



DIKES. 



Having now noticed all the rocks of the Crystalline series found 

 in Hingham, — Granite, Diorite, Petrosilex, and Diabase,- — and 

 as each of them is found in dikes within the Boston Basin, two of 

 them at least in Hingham, it seems fitting to present here some 

 special remarks upon the form of structure known under that 

 name, and to give an account of localities where they may be 

 observed. 



Dikes are igneous, unstratificd rocks, which occupy fissures in 

 the formations, and which have been forced up from beneath the 

 surface of the earth in a liquid or semi-liquid state, into the 

 superincumbent rocks. 



This molten material undoubtedly at first spread itself, as does 

 the modern lava of volcanoes, over considerable areas after reach- 

 ing the surface. As seen in Hingham, the rock of the dikes is 

 usually found only within the walls of the fissure that gave it pas- 

 sage, the decomposition and washing away of the hundreds of feet 

 of solid matter that once formed the surface having generally left 

 for our view only what is now seen within narrow limits. Often, 

 in forcing a passage through the invaded rock, masses of the latter 



