MUSEUM OF COMPARATIVE ZOOLOGY. 185 



Professor Thurston states that the mean of various analyses made of 

 this ore is about as follows : — 



SiOj 22.87 



AlA 10.64 



^ ^ 44.88 



FeO 



} 



MnO 2.05 



CaO 0.65 



MgO 5.67 



TiO 9.99 



Zn 0.20 



H2O and loss 3.05 



Total 100.00 



The ore on one side of the hill, where it has been most extensively 

 quarried, shows a dark, somewhat resinous groundmass, holding large 

 striated crystals of feldspar. The resinous lustre and greenish-yellow 

 color, as observed under the lens, are caused by the presence of olivine. 

 The olivine becomes more strongly marked on the slightly weathered s^^r- 

 faces seen on the faces of the quarry. Under a lens of high power, the 

 olivine shows clearly on the fresh fractures. The olivine in weathering 

 decomposes to a yellowish and reddish-brown ferruginous powder, leaving 

 the other constituent of the rock, the magnetite, well marked. The 

 magnetite decomposes moi'e slowly, and forms an incoherent mass after 

 the decay of the olivine. The rock gelatinizes with hydrochloric acid, 

 and yields a titanium reaction. A fragment allowed to stand a day or 

 two in weak hydrochloric acid yielded gelatinous silica copiously. 



A section made with special reference to the feldspar crystals shows 

 large porphyritic crystals of the latter enclosed in a mass of magnetite 

 and olivine. 



The magnetite forms irregular, more or less connected masses, making 

 a sort of sponge-like structure. Its rounded and iiTegular cavities are 

 filled with olivine, which also occupies the interspaces between the mag- 

 netite masses. The olivine is in rounded forms, which sometimes show 

 one or more crj'stal planes. It is cut through bj' numerous fissures, that 

 usually show a ferruginous staining along their sides. The olivine also 

 holds grains of the magnetite. Except the fissaring and fen-uginous 

 staining, the olivine is comparatively clear, and shows little signs of 

 alteration. 



The plagioclase feldspar shows well-marked lines of cleavage and frac- 



