103 



variety of hornblende. I think the feldspar is not always of 

 the same species, and the same is probably true of the horn- 

 blendic material ; but I have not investigated, or tried to 

 separate, the various rocks thus resulting. 



Dark-colored mica (biotite) is of very common occurrence 

 in the diorite series, and yet it is rarely abundant. It is more 

 characteristic of the diorites proper than of the granites. 

 These rocks are occasionally epidotic and chloritic, but neither 

 of these minerals can be regarded as characteristic. Like the 

 granite and petrosilex, they are poor in accessory minerals. 

 Thin seams of specular iron are not uncommon, however, and 

 this series carries all, or nearly all, of the argentiferous galena 

 of Newbury and adjoining towns. Prof. A. Hyatt has called 

 attention to the remarkable extent to which this formation, in 

 some parts of its distribution, has been broken and shattered 

 by some upheaving or plicating force, and the numerous irreg- 

 ular, angular crevices and fissures thus produced, filled with 

 what appear to be sometimes endogenous, and in other cases 

 exotic, veins of flesh-red feldspar, mingled, usually, with 

 coarsely crystalline quartz, and, occasionally, with epidote. 

 In many cases the fractures are, locally, so numerous that the 

 rock is entirely broken up into small angular fragments, and 

 has a distinctly brecciated aspect, the endogenous feldspar form- 

 ing the paste. The structure is such that if the paste were 

 removed the fragments would fall together, and fit as neatly 

 as the pieces of a puzzle. The best exposures of these feldspar 

 veins in their various degrees of complexity are on the south- 

 east shore of Marblehead. 



The large area in Sharon, Stoughton, and Foxborough, 

 colored as diorite on the map, may contain patches of other 

 rocks ; but Mr. Very and I have traversed it from north to 

 south, along several difierent lines, and found little or nothing 

 but diorite ; and hence we conclude that it must be mainly of 

 the dioritic or "mixed" series. The boundaries are largely con- 

 jectural, especially toward the east. The rock is for the most 

 part very fine-grained ; but in the hills of Sharon it is some- 



