The Monteregian Hills. 221 



The hornblende thus belongs to the class of basaltic 

 hornblendes, and not to the barkevikites as might be 

 expected. It contains, however, proportionately more of 

 the iron in a ferrous condition, together with somewhat 

 less alumina and a somewhat larger proportion of alkalies 

 than most hornblendes. The unusually high extinction 

 for a hornblend of this class which it possesses is probably 

 connected with the high content in ferrous iron, since 

 Schneider 1 has shown that the extinction increases with 

 the increase of iron in this state of oxidation. 



Pyroxene. — This mineral occurs intimately associated 

 and often intergrown with the hornblende, both minerals 

 frequently holding many inclusions of magnetite and 

 apatite. It is very pale-greenish in color, with no per- 

 ceptible pleochroism, but with a marked dispersion of the 

 bisectrices. It is usually hypidiomorphic, but is frequently 

 idiomorphic, showing a distinct cleavage parallel to the 

 pinacoids, but usually none parallel to the prismatic faces. 

 It belongs to the variety of diopside-like augites which 

 occur in rocks of this class. The extinction is high, 

 reaching 45°. 



Biotite. — This is deep-brown and almost identical in 

 color with the hornblende and is strongly pleochroic, 

 C yellowish-brown, and a deep-brown. It occurs inti- 

 mately associated with the hornblende and augite, and 

 also frequently as a border around the iron ore. While 

 usually present in comparatively small amount, in the 

 finer-grained essexite forming the summit of the moun- 

 tain it is much more abundant than the hornblende. In 

 this variety of the essexite both the mica and the 

 hornblende often possess a poikilitic structure owing to 

 the. presence of numerous inclusions of plagioclase, which 

 mineral also often penetrates the individuals of biotite and 

 hornblende in theform of well-developed crystals. 



Olivine. — This species is found in the finer-grained 

 variety of the essexite at the summit of the mountain, 



1 " Zur Kenntniss basaltischer Hornblenden yst, 1891, p. 579 



