36 History of Hingham. 



entrance a dike six feet in width, of porphyritic texture and par- 

 tially decomposed, its direction being, like the others, east and 

 west. 



The cove is about ninety feet deep, and is bordered on its south- 

 erly side by granite, having here and there more or less mixture 

 of trap. Some Melaphyr is also seen in juxtaposition with the 

 granite, and this rock also appears on the adjoining land near, 

 but to a limited extent. 



Martin's Lane. — On the right of Martin's Lane and just 

 beyond its termination, a dike may be observed within granite 

 walls, having an east and west direction and traceable 100 feet. 

 Its width is about six feet. 



JOINTS. 



Joint structure properly finds place here, as all the rocks of the 

 town exhibit it, and none more than the granites. 



Probably there can be found no reader of these pages resident 

 in Hingham who has not observed lines of fracture both in the 

 granitic and the sedimentary rocks of the town, as his eyes have 

 rested upon its numerous ledges. To explain these it will be well 

 to give some account of different kinds of joints that occur in 

 rocks, as they vary in character, have an entirely different origin, 

 and give rise to varied structure. 



The first to occupy attention, then, are such as arise from the 

 contraction by cooling, as in the case of igneous rocks, or by desic- 

 cation, as in the case of sedimentary strata. This contraction 

 results in cracks never parallel or intersecting, and are generally 

 short and not continuous. In some igneous rocks the contraction 

 tends to the formation of polygonal columns, which the joints then 

 surround and embrace. The best exemplification of this structure 

 is seen in the Basalt of the Giants' Causeway in Ireland, where 

 this structure presents the whole rock mass in beautiful prismatic 

 columns, each column separated into blocks having concave and 

 convex surfaces. They vary in dimension and are somewhat 

 irregular, but have been regarded by some as resulting from im- 

 perfect crystallization. There is, however, nothing of crystalliza- 

 tion in their formation, this being without doubt entirely due to 

 contractive action. Professor Crosby has mentioned a case where 

 the columnar structure was observed by him in the felsite of 

 Needham, but no instances of the kind have been noticed in the 

 rocks of Hinscham. 



The joints next to be mentioned are such as have now re- 

 ceived the name of Joints of Expansion. Almost all rocky 

 masses have, in addition to those of other character, joints, or 

 seams as they are often called, that arc approximately horizontal, 

 or nearly parallel with the surface of the ground. They may be 

 observed in any quarry. They divide the rock into layers, and 



