193 



oped in the more northerly and hirger ot" the two small quarries. 

 The joints of one system are phenomenally perfect, close, and 

 parallel, dividing the granite into almost absolutely plane sheets 

 varying in thickness from half an inch to two feet or more, but 

 mostly from four to twelve inches. The trend of the joints is 

 approximately N. S. (S. 5°W.), and tlic hade about vertical 

 (AY. O^-.H".) The granite thus, in a general view, simulates a 

 bedded rock, like sandstone, very closely ; and the surfaces of 

 the sheets are so plane and smooth and the grain so perfect, 

 that blocks suitable for building purposes are obtained with 

 remarkable ease, the joint-structure serving the same useful 

 purpose in this granite as in the Roxbury puddingstonc. It is, 

 of course, needless to dwell u[)on the obvious support which 

 this example lends to the earthquake theory of parallel 

 jointing.^ 



Irregular dikes and masses of the more finely crystalline and 

 micro-crystalline granite, and of felsite, are frequently 

 observed cuttinn; through the coarser fjranites and also the 

 diorite, precisely as in Cohasset. Perhaps the most interesting 

 exposure of these older rocks is that on the siunmit of Fort 

 Hill, where the diorite has been laid bare in grading the street. 

 Tiie diorite is very sharply and clearly intersected by numerous 

 narrow, branching dikes of a finely crystalline pinkish granite. 

 Tlie dikes are broken by slight faults and enclose angular 

 fragments of the diorite.^ On the north side of East Street, 

 between Andrew Heights and Kilby Street, the diorite is cut 

 by an irregidar dike of a gray felsite from eight to ten inches 

 in width. This proves on analysis to be a basic felsite, agree- 

 ing in composition better with syenite than granite. At many 

 points on Andrew Heights and along Kilby, East, and other 



' Proc. B. S. N. II., V. 22, p. 72^85. 



'-' Since the above paratcraph was written, tliis lieaiitiful and instructive exhibition 

 of tiie relations of the {granite and diorite has been entirely and rutlilessly obliterated 

 by thefurtiier grading of tiie street; a fact which every student of Hingham geology 

 will sincerely regret. 



OCCAS. PAPERS. B. S. N. 11. IV. 13. 



