24 History of Hingham. 



GEOLOGY OF HINGHAM. 



The geology of Hingham, particularly that of the northern part 

 of the town, though interesting, is of too abstruse a character to 

 be even partially understood except by those who have made the 

 rock-formations of the vicinity of Boston a study ; and its elucida- 

 tion will require on the part of the writer much reference to what 

 is exterior to the limits of the town. That of the greater portion 

 of its territory inland is more simple, exhibiting Granite as the 

 prevailing rock, but having some areas of Diorite, and occasion- 

 ally dikes of Diabase, which cut through the others, and appear 

 at the surface as black or dark-green rocks traceable often for 

 considerable distances, having a width sometimes of but few 

 inches, but frequently of several feet. Petrosilex is also found 

 associated with the granite, but in very limited exposures. 



GRANITE. 



This has been mentioned as the prevailing rock of a large por- 

 tion of the town. It seems necessary to first define what is meant 

 by the name before referring to its particular exposures on the 

 surface and its variation in character. Until quite recently geolo- 

 gists called all such rocks as were composed of quartz, feldspar, 

 and mica, granite ; using the term " syenite " to distinguish those 

 which had hornblende in the place of mica. When all four min- 

 erals were found together, the rock was called hornblendic granite. 

 The advance of the science of lithology has led to more strict defi- 

 nition. Now the use of the name u syenite " is restricted to rocks 

 composed of orthoclase (one of the group of feldspars), or ortho- 

 clase and hornblende, or orthoclase and mica ; while the essential 

 constituents of granite, as now defined, are quartz and orthoclase. 

 If to these mica is added it is called micaceous granite, and if 

 hornblende, hornblendic granite. Hence the rock of Hingham, 

 as well as of Quincy, is granite, and not syenite, as it is often 

 designated. 



Over the whole of South Hingham and the greater part of 

 Hingham Centre, wherever there are exposures of rock above 

 the surface it is granite, excepting only the material of the dikes 

 which are frequently found within it, and which will be hereafter 



