396 



Scientific Geology. 



only two miles north of the Connecticut line, and near the western 

 line of the town, has been explored in some places to the depth of 60 

 or 70 feet. I have already described it so fully as to render it ne- 

 cessary to add only a few remarks respecting its geological situation 

 and mineralogical characters and associations. It is most decidedly 

 a bed in a dark colored gneiss, which here dips from 60° to 70° west, 

 and runs N. 30° East, and S. 30° West. In immediate contact with 

 the gneiss, we find frequently lamellar brownish hornblende, which 

 is also disseminated to a considerable extent in the gneiss. 



The lustre of this plumbago is highly metallic. Its structure is 

 between scaly and fine granular. Sometimes, however, there is an 

 obvious approximation to distinct crystals ; though mineralogists are 

 not agreed that this substance has ever been found in such a state. 

 Judging, however, from specimen No. 1075 and from what the work- 

 men told me, I suspect that if crystalized graphite occurs anywhere, 

 it may be found at Sturbridge. 



There is another variety found at this locality, which is distinctly 

 fibrous j the fibres being from one to two inches long. On examina- 

 tion, these fibres are found to be composed of distinct lamellae, which 

 are sometimes so bent as to give the mass a fibrous appearance ; 

 as happens in certain varieties of mica slate : but more commonly 

 these lamellae actually separate longitudinally into very narrow 

 prisms, like prismatic mica. (No. 1074.) 



At this mine I noticed phosphate of lime in small quantity. At the 

 most southerly excavation, also, I noticed hydrate of iron, in a cross 

 fissure in the gneiss, and forming with the ingredients of the rock a 

 brecciated mixture. Vegetable relics are sometimes seen enveloped 

 in the mass. Haifa mile north of the meeting house in north Brook- 

 field, I noticed a similar breccia, forming a bed in gneiss a foot or two 

 in thickness : though here I saw no vegetable remains. 



In both these cases, I think we must regard this iron ore as having 

 been infiltrated into cavities in the gneiss, at a recent date ; and, 

 therefore, in fact, as an alluvial deposit ; although at Brookfield the 

 iron forms a distinct bed in the gneiss. But the rock contains abund- 

 ance of decomposing sulphate of iron, which, as we have already 

 seen, produces bog iron of exactly the same aspect as that above de- 

 scribed : and it is not impossible that from this cause a cavity in the 

 rock that was originally small, might have been much enlarged ; 



