32 History of Hingham. 



were torn off and enclosed in the molten matter, and it is not 

 therefore uncommon to find in Hingham instances of the enclos- 

 ure of granite within the darker trap rock of the dike. 



The name Trap has heen generally used to designate the dark- 

 green or black rock forming dikes ; but as it is now recognized 

 that different rocks of like appearance constitute the invading ma- 

 terial, it is necessary to be more definite in scientific description. 

 The dikes of Hingham as far as examined, with two or three ex- 

 ceptions only, are all of Diabase. 



In narrow dikes the rock has a homogeneous structure, as the 

 sudden cooling prevented a crystallization of its mineral constitu- 

 ents, but in those of any considerable width where the material 

 cooled more slowly, it is often porphyritic towards the central 

 portion, crystals especially of feldspar being disseminated. Upon 

 the invaded rock the action caused by the introduction of the 

 molten matter is generally more or less perceptible by a change 

 in its structure near the junction of the two rocks, and frequently 

 by the production of minerals along their margins. In Hing- 

 ham, Epidote is not uncommonly found as the result of this action. 

 Mention will now be made of some of the dikes which have come 

 under the observation of the writer. 



Meeting-House Hill, Main Street, South Hingham. — There 

 is a dike in the granite of this elevation but a few steps north 

 from the church which may be seen on the surface of the rock 

 and traced sixiy to seventy feet to the margin of the carriage 

 road. It is from five to six feet in width, and runs in a northwest 

 and southeast direction. Generations of men have come to the 

 temple here to worship, wholly unconscious that their footsteps 

 were over a record of events that took place millions of years 

 before man breathed the breath of life. 



Leavitt Street and Jones Street. — Between these two roads 

 on land of Mr. James Jones is a rocky hillock of granite about 

 equidistant from both, in which may be found three trap dikes 

 not far apart, one of which has the considerable width of ten 

 feet. To readily find these, proceed from the bridge that crosses 

 Weir River 700 feet in a southeasterly direction on Leavitt 

 Street, which will bring one to Mr. Alanson Crosby's house on 

 the left side. By passing to the rear of the house about 300 

 feet from the road, the rocks will be reached with their en- 

 closed dikes. The most northerly of the three is about two and 

 a half feet in width, the second, eighteen feet from the first, 

 is ten feet wide and exposed for a distance of seventy-five feet. 

 These two show well on the face of the granite cliff which en- 

 closes them. The third, forty feet from the last-mentioned, is 

 from three to four feet wide. This will not be readily perceived 

 without close examination, as it is only on a comparatively level 

 spot and obscured somewhat by surface soil. The direction of 

 these dikes is east and west. Two hundred and fifty feet, more or 



